It's very easy to recreate: just give yourself a hitscan weapon, type either noclip2 or fly in the console, go to the top of a tall room, then fire directly at the ground. I even have screenshots illustrating this:
Spoiler:I have no idea what version of GZDoom this started in, but it exists in the latest build (2.1.pre-508-ga8398e7) I've tried from DRDTeam. It may not even be a bug given limitations with the Doom engine, but this doesn't seem right to me so I'm posting it anyway just in case.