After update from 10.15.3 to 10.15.4 external displays doesn't work via docking station

[[Similar issue here. User with a 2010 Apple Cinema display, which involves three connectors: USBc for power, USB a>c adapter for a/v, and a UPTab USB-C Type C to Mini DisplayPort Adapter. As the user stated, it's not pretty, but it worked up until the 10.15.4 update.]]

I have the exact same situation. Although the Cinema Display is from 2010, it has only been used sparingly for the past 2 years and sat in the original box until 2018. I have a MacBook Pro from 12/2018 and the external Cinema Display was working perfectly up until the Catalina 10.15.4 update. I have tried a number of things I found online, including resetting the SMC, resetting the NVRAM, and starting in safe-mode, and then restarting. Also tried pressing Ctrl-F2 after each reboot; unplug/plug the monitor; swapped external usb-c connections around. During this time I was lucky enough to get the display to be detected by the system, however, once the display is disconnected again the problem reoccurs. Currently stuck trying to get it to work again - no luck so far. I opened an issue with Apple support - phone support stated that they would call back is an estimated 2 minutes wait - the call came in very late at night but I was unable to answer the phone in time before the auto caller hung up. I will be trying again since I now have a case#. Here is the Software version info for my machine, looks like I am at the latest upgrade, maybe it will be of help to someone:

System Software Overview:

  System Version: macOS 10.15.4 (19E287)

  Kernel Version: Darwin 19.4.0

  Boot Volume: Macintosh HD

My Apple LED Cinema Display disappeared after updating my MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports) to 10.15.4; but only after plugging in my FitBit Versa 2 to charge on another port.

(I use an Uptab USB C to Mini DisplayPort Adapter to connect the display).

Even unplugging the FitBit wouldn't restore the display. A reboot would temporarily, but it would soon start flickering off and then disappear again entirely.

I reset the SMC and the monitor came back, although plugging the FitBit in still caused trouble. I now charge the FitBit using the display's USB port instead of my MacBook Pro's and that's an acceptable workaround. Still, things not as solid as they were prior to the update. If it goes again I feel resetting the SMC and avoiding plugging in peripherals to charge directly into my laptop will be the solution until there's an update from Apple to restore things to how they were before.

I don't know exactly what you mean, Here is what I did. Under the direction of apple support:

I rebooted into the macOS Recovery screen

Logged in - takes you to the macOS Utilities menu

Connected to my internet

Selected Reinstall macOs from the menu

Went for a hike

1 - 2 hours later the process was completed and the system had rebooted and the external display was properly mirrored.

Once the external display was disconnected and reconnected the MacBook no longer detected the external display.

Not sure if this is helpful, but I found a workaround to my problem. I have the following device to hook up my display:

https://www.amazon.com/gp/product/B07D33RMRN/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

If I just plug it into my laptop with an HDMI Cable running to my monitor, it will not work. HOWEVER, if I plug my phone into one of the USB ports on this dock, it will work...

I have no idea what's happening and why this works, but it does 100% of the time. If you're having similar issues and you have a phone you can plug into one of the ports on your device, give it a shot?

SO I finally found something that works for my new 2019 13-inch MacBook Air again... yeah, it sucks I had to spend some money on it, but you know what, I finally found a working Dock that does what I need it to do after that stupid updates.

Anyways.. the dock I purchased is - https://amzn.to/2Y2mPZL

Only think that sucks is only accepts DisplayPort. One of my monitors has DisplayPort, the other was HDMI. So I purchased this adapter for the second one - https://amzn.to/2VtbzE3

I got it all set up and FINALLY everything works just the way I want it, plus my new Dock has more ports than my old one, so it is kind of like any upgrade anyways - plus I am using this all for work, so it's tax deductible.

Still thinking it's kind of crazy that Apple hasn't invented their own Dock they want us all to use, but I will be happy with my new toys as long as it keeps working on my computer.. and I will ALWAYS be sure to BACK UP NEXT TIME!!! :-(

Hope this helps you as well!

I have the same issue on Macbook Pro 16" 5500m, my Asus PQ321 4k display, which is same panel as the Sharp 4k display just shows black after the10.15.4 update. The problem is that the firmware I believe is causing the issue and I have tried 10.15.5 beta 2, beta 3, downgraded to 10.15.2, 10.15.3, 10.15.4...nothing works! The firmware after its been updated it wont go back and there is no way to revert even if you go back to 10.5.3 or earlier. I tried 10 clean installs, nothing..'

I'm talking to apple care now and waiting for their engineering dept to get back to me, doubt they have a fix though :(. I've tried everything on the internet, displaylink drivers, privacy, safe boot, reinstall, smc, pram/nvram but nothing works. I'm pretty certain the firmware update from 10.5.4 broke it, if there was a way to go back I'm 99% sure it would work.

I need to use my monitor or else why do I have this $4500 machine if I cant use basic standard computer things... 10.15.5 beta 4 is out, but I haven't tried and I'm kind of so defeated if it doesn't work I'll be sad. I ordered an eGPU razer x with 5700 XT, so I'm hoping that works and actually will run the 4k 60hz scaled a lot smoother and leave the dgpu for processing.

Very frustrating...

Following the 10.15.4 update my 2007 Viewsonic external monitors (connected via mini-displayport to DVI) began blacking out after waking my 2013 27" iMac from a 1+ hour sleep. Putting the system to sleep again and waking immediately usually fixed the issue, but was unacceptably annoying. All corrective measures offered in this and other discussions were unsuccessful, and the most recent update (10.15.5) did not resolve the issue. A 2019 LG monitor from another setup was tested (via mini-dsiplaypot to HDMI) and did not black out after waking from 1+ hour sleep periods, so I suspect that macOS Catalina stopped supporting (or altered timing of) an older type of video handshake (or perhaps there's an issue with DVI support).

I ended up buying new Acer monitors that are connected via mini-displayport to HDMI - problem solved - no more black outs after waking from sleep. Therefore, if others are still experiencing external monitor blackout issues I recommend testing a newer monitor. If successful then the older monitor(s) should be replaced as it's unlikely at this point that Apple will fix the problem for older monitors.

I was running 2nd monitor with 3rd party usbc-hdmi connected to 2019 mini until upgrade. Finally bought the $69 Apple adapter and got it working.

Try changing the ports. That worked and now I can use any port after reboot.

An additional reboot fixed it for me. Maybe not for everyone else.

After a successful downgrade (its possible but requires searching with google) don't forget to turn off automatic updates.

I purchased a CalDigit TS3 Plus to test if this would fix the issue, however the same behaviour occurs. This appears to be a display issue only, and not an issue with docking stations or hubs.

I can copy this behavior. For me it is happening on a LG Ultrawide connected via TB3 Dock and HDMI. Im on the MacBook Pro 16".

I tried already a couple of things: Plugging my dock in every other TB3, cold/hot plug. Reseted PRAM and did a safe boot.

I can verify my dock is working since my iPad Pro has no issues connecting to the monitor through it.

I also tried a rather different approach: Keeping the lid closed of the MBP while connected to the TB3 Dock. This actually results in showing the startup and login screen on the external monitor. However as soon as I am logged in, the monitor briefly shows a distorted image and then is black.

I cannot test it with another dock or adapter, as I unfortunately don't have one.

Does your docking station get its power from an Apple supply or a normal power brick?

What I continue to see is that 3rd party power sources don't play nice with the MBP anymore, even when the monitor is working fine. [https://discussions.apple.com/thread/251214002]

reset of SMC is a temporary fix. The same behaviour returned after a few minutes. After start of the screensaver, the external monitor turns off and never wakes up again. Disconnecting the external monitor wakes up the internal display. Reconnecting the external monitor too the MBP starts process of connecting of the external display, the screen turns itself off (like starting to build an extended display) but never gets to the point of actually displaying anything. The internal display stays off and the external monitor stays dark or starts to wake-up and turn off, wake-up turn off repeat.

I am no apple software expert, but its like the system detects the external monitor, starts or changes a driver setting but the driver crashes while setting up the new monitor. The system detects the crash, initialises the video driver, gets a signal that an external monitor is connected and voila, the cycle repeats.