Yikes! That is.......so bizarre!
But thank you for posting the solution. "Automatically choose" is the default selection for Ren'py and it causes issues for me as well - though nothing like what you saw. @_@ I just get weird flickers and flashes of dark areas.
This happens in every Ren'py game for me - not just Gilded Shadows. I always have to set the renderer to a specific one when I download a new Ren'py game.
O_O