I have a related problem with LG 32UK50T HD monitors (3840 x 2160) connected HDMI-HDMI (with certified 8K/"Ultra High Speed" cable). These monitors automatically go into "power saving shutdown" after a short time of no signal, e.g. "turn off my screen after 15 minutes" (new HP Pavilion laptop, Iris Xe Graphics, delivered with Windows 10 Home and just upgraded to Windows 11, resulting in installation of the Generic PnP Monitor driver). Once the signal from the laptop is shut off by Win11, the attached monitor doesn't respond properly when the signal is restored by waking up the laptop. It appears some sort of "wakeup" signal is sent, the monitor sort of wakes up, then decides there is no signal and goes back to sleep.
My workaround is to either disconnect/reconnect the HDMI cable or switch the monitor input from HDMI1 to HDMI2 and back again, after which the monitor is alive and well (until the next screen turn off).
I'm contemplating (re)installing the LG "Ultra HD Driver.zip" for Windows 10 I've downloaded from the LG support site, but haven't yet.