Ah, interesting. I'll have a play around and see if I can find what caused it. As I'm asking it to do something it's not really meant to, there are weird outcomes sometimes. Thanks for letting me know.
I did assume others have done the same hack, though I can't really see them at the minute. I could have swore Borksy had it, though I can't see it.
Interestingly, I was playing one of my old favourite Bitsy games today, and was surprised to see that had mouse movement: https://linda-nd.itch.io/day-in-the-life
Looking at the Bitsy changelog you can see Adam talk about it at version 3.6 : "bugfix: remove broken mouse controls." Maybe it was causing more trouble than it was worth. Recreating it to how it was would probably be a lot of work, but doing something similar to my simpler method might be cool. I wrote a little devlog about it if you want to understand it a bit more.