Similar problem...Big Sur, Mac Mini 2018 with two (2) LG 27" 4K monitors. I can only get one monitor to work at a time. I've tried a dual monitor configuration using USB-C connections for both and USB-C for one monitor and HDMI for the other. I've tried Detect Displays to no avail.
I had a similar issue with Catalina, but only at startup. If the 2nd monitor wasn't recognized, shutting down and restarting resolved the issue.
cryptodevice
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
From
Huntsville, AL, United States