This happened to us last week in Story too. Not done Story this week but I guess it aint fixed. Seems to be intermittent though, since we wiped on the attempt where the turrets didnt despawn, then tried again straight after and it worked as intended, turrets shut down.
For now, keep trying. Hopefully this thread will grab devs attention if they're unaware of this bug. They appeared to jump straight onto fixing the Kephess bug, so hopefully the same will be done with this