12.3.1 update did nothing too resolve this issue for me. My workaround is not particularly elegant but... Mac mini with Lenovo 27" plugged in via USB-C, designated as 2nd monitor (no menubar) - first monitor LG 4k 27" plugged in via HDMI—at startup I have to unplug the 2nd Lenovo first, then startup... wait for the mac to boot up, then connect the USB-C plug for the 2nd Lenovo monitor. If I try to boot up with both monitors plugged in, only the 2nd Lenovo monitor shows up but of course no menubar, and the other monitor is black, and doesn't appear connected. No other sequence of steps works, and I've tried everything. This has worked for the past 3 weeks, but now I have to hard stop the first boot up because nothing happens until I re-boot after a hard stop to get that first monitor to show!!! Any idea how I can get this info to Apple?