Thanks for trying the game and the bug report.
I can't see to reproduce your issue, jump works in both fullscreen and windowed mode for me for all 3 keys.
have you tried all 3 keys? W, K or space?
I've only tested on a qwerty keyboard with US settings, are you maybe using a different keyboard layout?
if so can you tell me which one? so i can keep that in mind for other submissions