This was happening due to a bug in the GZDoom 3.7 series. Try running with the just-released GZDoom 4.0: https://forum.zdoom.org/viewtopic.php?f=1&t=64188
Yep, it's fine now! Haha, remarkable timing there, eh?
Oh crap, new bug. The game crashes whenever I try to load Doom 2 MAP16.
VM execution aborted: array access out of bounds. Max.index = 778, current index = 2491
Called from FriendlyMapCompat.HideLine at friendly.pk3:friendly_zscript/mapcompat.txt, line 36
Called from FriendlyMapCompat.RunMapCompat at friendly.pk3:friendly_zscript/mapcompat.txt, line 29
Called from FriendlyHandler.WorldLoaded at friendly.pk3:friendly_zscript/handler.txt, line 365
That's the only map I've seen with this error so far.