May be related to MinecraftForge/MinecraftForge#7766, though that happens on exit/rejoin, still have to confirm.

Can this be reproduced on 0.0.11? I can't find anything of this nature on either Forge or Spigot

See https://bugs.mojang.com/browse/MC-224729

Should be fixed for Forge via mixins, no solution for spigot/paper yet atm.

Closing this for now as there is no possibility for a solution on our end.

FWIW, I think this used to happen on my server in 1.12.2 when using worldedit's regen command. We're on 1.16 (without OTG) now, and it's certainly a good bug to fix, but it may not be new.

It's definitely new, it does not happen in 1.12.2 without regenerating chunks with something like worldedit as per your example.

Should have mentioned that it happens in a line indefinitely across the entire world:

This is unfortunately a vanilla bug, and it happens most often when the world is closed and reopened. I don't think there's any good way around it.

Without looking into it I don't think it's possible to come to that conclusion. I'll have to make a reproducible example.

The above screenshots were taken on a fresh world which hadn't been closed and reopened.