I casually replayed the game. It's the same GameTop copy that I speedran before. But when I reached the boss, his level ended almost after half a minute, as opposed to just a few seconds like it happened before. I updated Windows 10 since my speedruns, but other than that I have no idea what could cause this change. And I still haven't figured out how the boss is meant to be fought.

Oh, and when the level ended, the camera randomly went under the floor. https://64.media.tumblr.com/47bf79e75c614b2ee2de4ed089f07eb1/20c323c83089d1d0-78/s1280x1920/5c47610e58fe8a606ee096c46f61b381f044c62e.pnj