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.
