I've tried one of the downloads and I can't reproduce the issue, unfortunately. The Github thread mentions updating video drivers, but I'm not sure what that could solve as it concerns missing files in the Ren'Py directory. The fact that you have the issue with another game (also Ren'Py?) is however a strong indicator of something interfering on your end. Game optimizers or anti-virus might be the most realistic option, but you already mentioned don't run those.
If you learn more about the issue, I'd be interested to know more about it.