J Leslie Turriff composed on 2025-08-23 20:47 (UTC-0500):
| @20:34:37 CyllarusCentaur@chestnut | wd=/home/CyllarusCentaur | $ xrandr|grep 'Screen|DP-|HDMI-|3440x1440' | Screen 0: minimum 320 x 200, current 3440 x 1440, maximum 16384 x 16384 | DP-1 disconnected (normal left inverted right x axis y axis) | DP-2 disconnected (normal left inverted right x axis y axis) | DP-3 disconnected (normal left inverted right x axis y axis) | HDMI-1 connected primary 3440x1440+0+0 (normal left inverted right x axis y axis) 800mm x 334mm | 3440x1440 100.00*+ 84.96 49.95 | HDMI-1-2 disconnected (normal left inverted right x axis y axis) | DP-1-4 disconnected (normal left inverted right x axis y axis) | DP-1-5 disconnected (normal left inverted right x axis y axis) | DP-1-6 disconnected (normal left inverted right x axis y axis) | rc=0
which indicates that it is in its preferred (native) resolution.
(I won't be using xrandr to modify any of the zillion settings shown on the info page; hardware stuff makes my head hurt. :-) I never should have messed with Monitor & Display in the first place.)
If it's still not working, I suggest simply switching to a DisplayPort cable. DP was /designed/ for computers, HDMI for TV. DP has functionality in PC environments HDMI cannot provide that sometimes makes a difference, e.g. MST[1]. I suspect also a lot of Linux devels don't have 3440x1440 displays to test with, so must rely on actual user reports to know if trouble with extra-wides sneaks into something.
[1] https://en.wikipedia.org/wiki/DisplayPort#Multi-Stream_Transport_(MST)