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.
Unfortunately... we've come up with nothing. As we continue updating with the ren'py engine... we're sortof stuck hoping the issue resolves itself. (Probably not what you want to hear... but at this point it's the best idea we've got.) T.T