After Monterey 12.3 upgrade, can't run 2 usb-c monitors

Indeed I've seen it with LGs monitors on 3 different Mac mini 2018, all using USB-C connections and one of the displays "vanished" after upgrading to 12.3.

My take: Macos cannot distinguish the monitors, because LG monitors (at least those that I have 32UL750) Don't present a serial number and thus 12.3 is only "recognizing"one of them (the one that "answers" first).

Macos up to 12.2 had a terrible time properly identifying the monitors and they would often swap on boot or sleep (I have my own keyboard shortcut to force them to swap). Again because there is no way to do it otherwise.

If they have "fixed" this swap issue, there could be an explanation for the whole problem, if monitors are indeed "equal".

We have 3 minis back in the game by switching one of the monitors to HDMI and used the following procedure to have it recognize the monitors properly:

• Shutdown
• Leave the HDMI connected and remove the USB-C cable from the mini (not the monitor as the cable may be active)
• Start, check the monitor, screen size and refresh rate. Shutdown
• Now power the second monitor and connect the USB-C cable
• Start the machine.

On all 3 minis both monitors came back.

I will try the procedure with 2 USB-C later, but for now I have to get back to work.

The 2018 Mac mini has four Thunderbolt/USB-C ports on two Thunderbolt busses. Make sure the displays are not using the same bus, i.e. on the two left or two right connectors. Plug one display into one of he left two connectors, and the other into one of the two right connectors.

If they are the 4K LG displays that Apple used to sell, they come with both USB-C cables and Thunderbolt 3 cables. The Thunderbolt cables should have lightning bolt icons on the plugs. Try the Thunderbolt cables instead of USB-C.

Also, if they are the LG 4K Thunderbolt displays, they will have two Thunderbolt ports each, so you could try daisy-chaining them from one computer port using Thunderbolt czbles.

They don't read threads on this forum, and the Apple Store will not be able to fix anything. Only workaround at the moment is to boot w/ 1 screen on the native HDMI port, then plug the 2nd one via USB-C once booted.

The issue has been lurking for months, I can find posts dating back to January from people who tried the beta version on their mini. It's obvious the relevant folks at Apple are unaware. Best is to open a bug report, like many of us did => Feedback - macOS - Apple

The more the merrier, only way they might eventually notice there's a problem.

I have same problem with two monitors LG 4k 27ul550-w. Just change DisplayPort Version from 1.4 to 1.2 in monitor settings - Settings->General->DisplayPort Version->1.2

Lowering the DP version to 1.2 (on monitors that have the option) appears to get around this problem. I had been unable to use my (single) LG monitor over USB-C/Displayport with my 2018 Mac mini following the 12.3 update until I made this change.

I had this problem on my 2018 intel mac mini after upgrade to Monterey 12.3. One BenQ 4K monitor connected with HDMI, another identical one connected to the USB-C port immediately adjacent to the HDMI port. USB-C connected monitor would not function - not detected - after upgrade to 12.3. I moved the USB-C cable as far away from the HDMI port as I could. Presto! It works. Maybe that connects the monitor to a different bus?

hcsitas, citation please? You've asserted that twice now, but the specs say nothing about an HDMI being mandatory, unless using three displays.

medium-cool, +1 for filing a bug report. If you have a time-machine backup you could also try reverting back to before the update.

Some good and some bad news...

A friend that has 2 minis 2018, both with dual displays, reported to me today that his setup (HDMI+USB-C) stopped working when he powered on his machine (cold boot). This time the HDMI display was not working. Mine was on sleep and returned ok. I attempted shutdown/boot and the same happened to me, but with one mild different: the icon arrangement was not showing the icons that were supposed to be on the other (blank) display, as if the other monitor was there. I tried powering the HDMI off and on and the icon arrangement changed, showing all icons now (overlaying from both displays).

The good side: my take is that it's not a hole that was plugged, but rather some bad bug indeed. If we need to see this fixed, we *all* must file bug reports to make them see the issue. The Mini 2018 is still being sold by Apple, that give us hope.

Also, for the shutdown/boot, once it's working, if you do the following it's much simpler to get it back into the game with a single pass:

• Before powering the mini, turn the USB-C monitor off (monitor power switch will do it (see bellow)
• Turn your mini on, and let it boot solely on the HDMI monitor
• Turn the USB-C monitor on

We did this a few times and it worked. It's not ideal, but allows one to consistently get a working system on every boot... or at least until we find yet another "detail".

As for powering the display off, if your monitor uses a mechanical switch, I recommend adding an extension cord with a power switch (many cheap options on Amazon). You don't want excessive wear on the monitor switch.

Tdbozarth wrote:

I have had two calls w/Apple support so far yielding zero results other than 1) being abandoned by senior advisor who scheduled a follow-up meeting and then ghosted me

Interesting as I had the exact same experience. They didn't call me back at the prescribed time, and when I emailed they said they "were busy with another customer". When I tried to pick a new time they ghosted me.

With much effort, I downgraded to 12.2.1 and both of my LG thunderbolt monitors are working again. I will wait for 12.3.x, or whenever Apple acknowledges the problem and fixes it.

This is clearly a bug of macOS 12.3 because before this version my 2 external monitors were working properly. The temporary solution is that every time I reboot I will use the DP port along with the USB-C to activate the 2nd monitor then unplug the DP port.

Upgrading to 12.3.1 resolved my issue with 2018 Mac mini

The 2018 does support dual displays staggered on the TB/USB-C ports.

Like you, I've had dual Samsung monitors connected to the TB/USB-C ports of my 2018 Mac Mini for years.

From the number of 12.3 update issues on the board, something is not right and we need to let Apple know.

Feedback - Mac mini - Apple

Feedback - macOS - Apple

“My take: Macos cannot distinguish the monitors, because LG monitors (at least those that I have 32UL750) Don't present a serial number and thus 12.3 is only "recognizing"one of them (the one that "answers" first).

The best explanation by far for the mystery issues! However, if true, it shifts the burden on LG to fix (unfortunately!).

Yeap, same here (and with 2 friends with similar setup): need to boot with HDMI only and then turn the USB-C monitor power on. Sleep "kinda" survive. Once I faced a "monitor role swap" (common with 2 USB-C monitors) with a lost of resolution setting (true 4K -> "default" on both monitors). Also, I've seen the dock reverting to the default config (but I guess it's not related).

The good side: it's really a bug!!!