Hello!
This is not a bug I've ever seen before and not one any of my beta testers experienced. I believe it's most likely a compatibility issue between the Ren'py game engine itself (not my game specifically) and your machine. One person in this thread did experience something similar (you can see it a few posts up) and while I have never had a glitch on this magnitude, I do sometimes get small visual errors in Ren'py games.
You can try to resolve it by opening the game and hitting shift+G.
This opens the graphics menu where you can directly select a graphics renderer in the upper left corner. You can try to open this menu and selecting one of the two renderers instead of letting it "automatically choose" and see if that prevents this from happening again. (You may need to test it to figure out which one works best.
I hope that resolves the issue!