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

Downleveling a Dell UP2718Q from DP 1.4 to 1.1a (only option) has me back working on 2 USB-C<->DP monitors on macOS 12.3. The other Dell only supports DP 1.2 which explains why it worked in singleton mode more than the newer Dell and had me just about ready go beat the dog. ;-) Think I'll run down to the local Apple store and beat them up. ;-). Many thanks to the folks who initially made this discovery & suggestion.

"Maybe that connects the monitor to a different bus?"

I believe that's the case. Regardless, moving the cable to the port that is farthest away from the HDMI port was the only thing that worked for m.

Hoo-boy. An update: I have the (same) 12.3 installation working now with both monitors (eGPU + HDMI), but I'm not sure exactly what I did got it working again. Here are the steps I performed, to the best of my memory:

1. Intending to set my Mojave partition as the startup disk, I noticed it was absent from Startup Disks in 12.3's System Preferences (perhaps because that volume is on an HFS+ Journaled USB3 external drive?). So to reboot into Mojave, I had to hold down the Option key on startup.
2. When Mojave booted up again (after a double-boot sequence), the Apple logo actually started on the main/eGPU screen, with the HDMI output blank. At the end of the boot sequence, the eGPU screen went blank, but the HDMI screen still didn't show up.
3. I then forced shutdown by holding the power button. Before restarting, I pulled the eGPU thunderbolt 3 cable, leaving only the HDMI connection plugged into the Mini, and booted up again. I actually meant to boot into Mojave again, but forgot to hold the Option key down this time, so it actually booted into 12.3 (since that was still the "blessed" system). To my surprise, this time the boot and login screen showed up on the HDMI monitor, and completed bootup successfully!
4. I then plugged in my eGPU cable. Now, both screens were visible, though reversed in orientation and arrangement to what I wanted, but I was able to reconfigure things without issue.
5. I then retested with another reboot into 12.3, this time leaving both screens connected. This boot sequence went as I'd expected before (and what I saw during my "test run" last night): initial Apple logo appeared on the HDMI screen, but then both monitors displayed, with the login on the main/eGPU monitor I'd last specified. After logging in, both are now working as expected.

Hopefully this configuration sticks (I always set my Energy Saver prefs to never sleep the displays or the machine). If I run into issues again, I'll try to remember to report back here. I realize this thread was started with people having issues getting *non*-HDMI displays to work, but my ordeal seems to point to something just being generally wonky in 12.3's display detection. If people have an older backup they can reboot into, it might be worth trying the steps I performed to get this working.

This is definitely an issue, Apple shall release an update to fix it because it's real annoyance.

My configuration is one 4K through HDMI and one 4K via USB-C, the HDMI one failed to be turned on after each restart. And my tested workaround:

1. Full shutdown to prevent electricity shock, then disconnect the USB-C cable from the port on the mac mini connecting to the good one.
2. Turn on the mac mini, now it drives the HDMI monitor to work.
3. Leave the login screen on, re-connect the USB-C cable.
4. Now both monitors work.

The point is to let the machine drive the "bad" monitor to work first, then light up the others.

And disconnecting the power cord doesn't work because mac os somehow still can recognize the monitor if the data cable is still connected on.

I believe the 12.3.1 release also seems to have fixed my issue. My HDMI monitor has stayed working across multiple reboots and monitor power cycles after updating to 12.3.1. I will verify the monitor still works properly in the morning (I normally turn the HDMI monitor off overnight).

I still find it odd that my experience seemed the opposite of most others' (my eGPU output through multiple adaptors was working fine, but built-in HDMI was failing on 12.3), but I'm happy Apple put out a dot release fixing this (and apparently multiple other bugs) relatively quickly.

You should set DisplayPort version to 1.2 but connect both monitors using type-c. At least it helped me.

Per the spec, one display must be connected over HDMI for a dual-monitor configuration. I understand your 2 USB-C combo worked in the past but that’s no guarantee it’ll work in the future, especially if the spec doesn’t allow it. Try it with one over HDMI and the other USB-C? If it works, you can file a report with Apple, maybe they’ll fallback to what worked before.

So things kinda work with the HDMI to DP adapter, and I assume it kinda works with straight HDMI as well (I already use all my HDMI ports on my monitor for other stuff hence the need for the adapter for me).

"Kinda works" because the setup doesn't survive a reboot. The only way to make it work is to boot with the HDMI monitor plugged and the USB-C/TB monitor unplugged, then plug the latter once macOS is fully booted. On the bright side, sleep does work, and I hardly ever reboot my mini, so I can live with that, for now. Far from ideal, obviously.

The DP 1.2 "trick" only seems to be working with a limited number of displays--my 2 Dell displays can only do 1.2 anyway, and only 1 screen gets recognized if both are connected via DP.

The only way it works for me: connect a single screen via HDMI, boot, then connect the other screen via USB-C/DP once macOS is fully booted.

I've seem reports with dual Samsung too, so it's not limited to LG monitors. My guess it's the way Apple decided to distinguish the monitors does not take into account the fact that whatever they are using don't account for "homonyns" (for a lack of better word here). When I was playing with the display swap problem I noticed that all the internal identifiers for both monitors were 100% equal thus impossible to distinguish. For a while I've been using my keyboard shortcut to swap the monitors whenever I reboot or wake my Mac mini and the displays were reversed.

Let the monitor manufacturers fix it? Hard to believe they will do it. How do you expect LG to fix this:

The SERIAL is "BLANK" (looks like a cheap solution). There is no way to provide a patch that will install a serial or other mean of identifying the monitor. However, it's pretty easy for Apple to synthesize a serial or identifier based on the port/path of the connection. Just don't do it the way Windows do it, as it create yet a ton of other problems.

The sad note is that I've seen similar problems with Linux: the device in the class that "replies" first gets the role. Happens a lot with network cards. So a OS only solution may be complex too.

At least we can have a working configuration, albeit not perfect.

I have seen many folks mention LG as the culprit. I have the same problem with my 2 Dells (Dell's displays use panels from AU, Samsung or LG, but the rest is built by Wistron, formerly part of Acer, it's not designed/built by LG).

Was working like a charm until the upgrade, so it's not like it ever was a problem... until now.

Long story short, it's not a brand specific issue, the Mac Mini w/ 12.3 is unable be able to drive 2 monitors off TB/USB-C, period.

Dell U2718Q & UP2718Q here with same problem after 12.3 upgrade.

Circumvention is to run one monitor on HDMI. Lots of winking and blinking now. Not happy.

Had been working fine up until the upgrade except for occasionally getting right & left reversed as someone else wrote either on this thread or the other one.

You have two displays, the spec mandates one on HDMI. That said, it could be a bug that’ll get fixed in the next release or a long-standing bug that’s been plugged in the current one. Both are possible and have happened in the past. Your best option is to file a report. Before that, see if strictly following the spec still works. Good luck!

I have this issue too and thanks for suggestions.

I got both monitors to work temporarily by connecting both HDMI and USB-C/DP into one monitor and USB-C/DP into the other. However, after shutdown and startup the dual doesn't come back unless I start messing around hot unplugging and plugging.

I've tried booting with one HDMI and one USB-C/DP as well as booting with each monitor connected to USB-C/DP on different buses - nothing.

Looks like I'm back to single monitor until a fix as I don't want to mess around with the cables every time I fire up!

Both my monitors are DP 1.2, only 1 will work at a time, so it's not a 1.2 vs. 1.4 issue.

I tried a TB to 2x DP adapter, 2x USB-C to DP adapters, you name it, no dice, only 1 display comes to life at a time.

I just bought a HDMI to DP adapter on AMZ to plug one of them via HDMI and leave the other on USB-C, I'll report here if it works.

I suspect the introduction of universal control in 12.3 required significant changes to the graphics/display management layer, and we're all in a what Apple likely considers a fringe case, i.e. not a setup tested before release.