This seems to have started appearing with the most recent GZDoom, currently investigating.
This ended up being due to a bug in GZDoom itself, which has now been fixed. The next official release of GZDoom, whether it's 3.7.3 or 3.8, should contain this fix. If you don't want to wait until then you could try downgrading to an earlier version of GZDoom like 3.6.*