UESC's De spawning

I havent been able to recreate it but it has happened twice now. Once on dire marsh a day after launch when completing a quest to take out the commander for an item. I was clearing the area for 5-10 minutes after a different uesc wave. After starting to shoot at the commander squad they disappeared after about 2 minutes of a firefight.

The second time it happened was just before starting to write this. There was a firefight between me and a group of runners. The fight was slow and I used the UESC as bait. after about a 5-10 minute firefight I looted everything without being seen and then went to fight the UESC that was in the area. They de spawned.