Digital is correct - small factions with only one province tend to try to escape and simply vanish without a fight. I have seen is happen to the Swedes, the Norse, and the Portugese. Add to this the Serbs (happened to me when once I invaded with the Bulgarians) and the Armenians and I wonder if we have a pattern - is it almost a rule that when you invade a small faction early in their original and only province, they will simply try to escape to nowhere? Also, I have never seen it happen to the Irish, and I wonder if this has to do with whether you invade by sea or land. Perhaps in the case of the Irish the AI realizes that since there are no neighboring provinces, it has no way to go and stands and fights, but with the Serbs and the Portugese who have land borders it erroneously believes that an escape to a neighboring province is an option?
It happens rarely in MTW VI, never in vanilla MTW, and often in XL, so there must be something that VikingHorde has done to small AI factions behaviour in this reagard, even if not consciously. Would love to see this topic moved to the Mod forum to see his answer.