I found the Issue! I was using the Z key to jump, ya know, cause it's the correct one. But actually, for SOME reason, the J key also allows me to jump. And for an even MORE confusing reason, jumping using J allows me to run and jump to the left. I have NO idea why that's happening but hey! I finished the game so yay :D
(Also, did musky rat survive? I hope he did T-T. Poor boi needs some headpats!)