I also have this problem. I have three monitors in Nvidia surround giving a resolution of 5760 * 1080.
Amnesia and Amnesia:Justine's mouse movement worked perfectly - they are excellent games in ultra-wide resolution. I was disappointed to find that AMFP has this problem.
Vertical mouse movement is too quick, horizontal is too slow, so the ratio calculation must be wrong (or perhaps this is now fixed at a 1:1 ratio). If it can't be altered, perhaps we could have an extra multiplier parameter in the user_settings.cfg so we can specify the ratio?
If this could be corrected that would be appreciated (i'm not playing the game until i can resolve this). If i find a workaround I'll post back here