Hello,
As stated recently I had setup a Rock5B with Jammy/Mutimedia and had issues with a 2160P / 3840 x 2160 screen.
Initially bootup would show a flickering screen.
Adding the custom resolutions via cvt/xrandr and setting line in boot env: "exrtaargs-video=HDMI-A-1:3840x2160@60" fixed that issue
However the initial login screen and logout screen resulted in a vibrant flickering background .
This was on a Sceptre 50" TV and other perihperials would normally work fine.
This took a while to figure out because I was confusing my active desktop provider and Gdm was being used:
https://askubuntu.com/questions/912052/how-do-i-change-gdm3-login-screen-resolution
However a similar issue could arrise if using lightdm and this goes over changing the lgoin screen resolution also:
https://askubuntu.com/questions/73804/wrong-login-screen-resolution
Posting this just in case anyone else is having this odd issue as it was quite an eyesore.. Thanks!
