Oh this is a rendering error, hold SHIFT+G when you're launching and try a different renderer. (This usually occurs due to an incompatibility with something specific to someone's hardware/software specifics in their PC.)
Still working on it, but you're sure your graphics drivers and renderers are up to date? I know mine sometimes fail to update and I have to update them manually.
Well we're asking on the ren'py discord hoping someone wiser there can help, at this stage were at a bit of a loss, but we'll let you know once we figure out what it is.