Pierre Couderc composed on 2021-07-16 08:34 (UTC+0200):
Before reporting a bug, maybe it could be too a question of driver...?
There are two X drivers for Intel IGPs capable at least theoretically of
supporting 3840x2160:
Intel DDX, provided by xserver-xorg-video-intel
Modesetting DIX, provided by xserver-xorg-core
The Intel driver has been essentially in maintenance mode for the past 7 years,
its main reason for keeping it maintained being it is the only driver available
supporting ancient Intel IGPs. The only way to tell if it works better on newer
hardware is to try it. Don't expect much from anything as new as yours using the
Intel DDX instead of the Modesetting DIX.
Either of the two drivers can be specifically selected via a .conf file in
/etc/X11/xorg.conf.d/, e.g., assuming xserver-xorg-video-intel is installed:
# file: 15-ddxdrv.conf
Section "Device"
Identifier "DDX"
Driver "intel"
# Driver "modesetting"
EndSection
selects the Intel DDX, blocking X from attempting use of the Modesetting DIX.