Documenting here in case somebody else runs into this issue:
When selecting borderless windowed mode at native resolution (for me 3840 x 2160), the game renders in borderless fullscreen as expected, but mouse movement is constrained to the top left ~1/3 of the screen.
I believe it's a display scaling issue; to fix simply close Underrail, change your display scaling to some other percentage (i.e from 150% to 175%), and then change it back to your desired scaling.
I'm not sure if this is an intermittent issue, first-install issue, or something else. Cheers!
