Horizontal and vertical mouse sensitivity seems locked to aspect ratio.
They are out of sync when running at 21:9 aspect ratio (2560x1080), it seems to works fine at 16:9 (1920x1080) which I assume is the default aspect ratio.
Changing in-game mouse sensitivity has no effect on the disproportionate ratio between sensitivities.
at 21:9 the horizontal sensitivity is lower than the vertical compared to the default 1:1 at 16:9 aspect ratio.
Needs a fix to make them aspect ratio agnostic.