Hi, thanks for the compliments! I haven't been able to recreate this bug myself, but I only have Windows currently accessible to me. I'll try adjusting the angle slightly and we can see if that helps, as it might be some obscure calculation bug in GZDoom (which is unfortunately a bit beyond my ability to fix but maybe I can work around it)
After a bit of experimenting, I have determined that it only happens with the flatpak version available in Linux Mint's Software Manager. I believe it is because the flatpak is missing a dependency (which somehow only affects this very specific part of the game). The "bug" is resolved by using the deb version available on ZDoom's official website.