There is an issue with Skealings. Their heroes spawn litterally inside the settlement, and thus brake it. We cannot double click it for diplomacy, it cannot be attacked. If you ever wage war to them, you will see your allies stacking up front of their Settlement, and you won't be able to do anything about it.
Alas you have the mod to trade any sttlement. If you can manage to make peace with them, you exchange with any other settlement elsewhere and you can finish them off, or do it beforehand. It also prevents confederation if you play as a Norse.
Finally the same issue can happen with some other factions when you liberate them. Vanilla Markus Wolfhart in Old World map. SCM's Marienburg. I don't know if it's the case for any other factions. But this topic is about Skealings as it is from this mod unlike the others. And I had the issue on the Vanilla map (IE) as well as on Old World and IEE.