Act 4 - Whakapanu Island - Crab boss despawn

While exploring the area I found the entrance to the crab cave the boss appeared , shifted trough terrain and got locked on top of the entrance (immune to damage), I tried logging in and out and getting to town and back but it just dissipated from the map after that.

Same, crab spawned, my minions started attack it and he ran back up into his cave and I couldn't fight him.