While I was grinding in Ant Tunnel Park, there's an Evil Eye that went downwards straight through the ground and its location was getting all glitched up. It kept happening, it didn't happen once. I'm sorry for not getting a screenshot of it, but I hope you all can understand what I'm trying to say.
This is a known bug already. Sometimes mobs drop through platforms (or possibly through the ground) because of them being either near or slanted ledges. Attacking a mob while they're falling endlessly can get them back in place. Leaving the map - this only works if you're the only person in the map - will make the mob go back to where it dropped to. If so, from leaving and re-entering, you can possibly hit them before it drops down again.