Oh no... Personally I didn't notice anything wrong while testing it in the browser but since that's the case for you, maybe you can try the executable downloads if you're on Windows (I may try exporting for other OSs too but I can't test those unfortunately). Also, I assigned Space bar to jumping in addition to Z in the "-updated" uploads of the game in case the issue is specific to the Z key. Those updated versions are slightly improved after the end of the jam, though, so I don't know if it's alright to rate the game based on them.
I'm glad you liked the art and I appreciate your review though. I expect myself to draw better art at the next jam, as I've learnt a lot here and honestly I got only half of the actually planned art for this game done.