It is my custom to lock the screens when leaving my desk (CMND-CTRL-Q). this brings me to the login screen with my wallpaper in the background. after a while the system goes to sleep (anyway the screen goes dark and I have to unlock using TouchID. But sometimes, the login screen is displayed for hours and the screen never goes dark. I have a friend with nearly identical setup (just one Apple Studio Display) and have seen this happen on her system as well.
I usually shut down and restart when it happens.
Any idea why this would happen? I find the mishmash of screen and energy related System Settings to be very confusing.