Hey,
as far as I know swarms can normally only attack the turn after they surround the victims, at least this was the case in my previous run.
Now it seems they attack once they reach you, I think this is a bug?
Happend with rats swarms, locust swarms and vescavor swarms.
If it works as intended please ignore my post. 
I think they changed the how they work in the TT rules. Swarms are supposed to do damage at the end of their movement. Any targets in the area of the swarm takes damage AFTER they have finished movement. So I think it was changed because that game was designed as a real time instead of turn based.
I agree that could be the case here.
Tbf in the previous version you could easily kite swarms to death in turn base mode so I guess it is intended. ^^