[2020] Ancients Gameplay issue

Yea, this is an issue I've got fixed for the next version. The problem arises because the dungeons are being 'uncovered' (usually in coastal biomes that have deep water trenches). Sorry about the issue. I know it's super annoying.

Not a great solution but you can use the debug console to kill enemies:
Hit ` or ~ to open the debug window then type "kill -e" without the quotes. That'll wipe out these annoying baddies that have spawned.