Rare Essence mob spawning outside of map.

On my last map I found a Rare mob trapped in Essence that spawned in a spot that is unreachable, screenshot below. It looks like a small "room" generated without a way to actually access it. The only path is blocked by a wall...

I went around the whole map but no path leading there apart from the one blocked in the picture. So i wasn't able to finish the map.

Update: I tried to run the same map " OASIS " with a new waystone and now a pack of normal mobs spawned outside of the map in an inaccessible area: