Recently, I've been seeing a lot of posts complaining that the dungeon habitat gets stuck once moved onto an island.
This is a known bug.
Apparently, social point designed habitats to never leave the islands, and now they are having trouble getting this habitat to go back where it belongs. The word is that they are trying to fix it.
For now, the answer is to contact social point and they will put it back for you.
I have an email out to social point and a few threads at their forum. I'm trying to get a lock down on the easiest/best way to do this. Keep an eye out here for updates.