Hey,
I can't seem to be able to play in 4K, that is, 3840x2160. The lower mode 3200x1800 doesn't show either. The latter mode would be good due to tile size. Perhaps you're using an arbitrary limit as a sanity check? Note that 8K is coming eventually.
in b4 system problems: all modes work at 60 Hz with DisplayPort. Other games show them, even a Falcon 4.0 derivative.
[edit 2x]
It appears that only modes with less than 1500 max height are allowed.
The highest working display mode is "2560x1440 (16:9)". Here's what happens at "3440x1440" [no suffix in Underrail video menu].
Higher modes make the statusbar show at the wrong position, and treat bottom screen edge as if it was right below the statusbar. So you get:
[ top physical screen edge ]
[ gameworld ]
[ statusbar and bottom scroll position ]
[ more gameworld ]
[ bottom physical screen edge ]
Perhaps the display size limit is applied inconsistently as well, allowing lower (relatively speaking) resolutions but clamping statusbar position to an even lower limit?