Not sure if this was a bug or intended, so posted here first.

On a purge mission, got it down to around 10% remaining enemies and in the course of a couple minutes it climbed back up to 50% as several of the rooms I'd already cleared respawned. Fortunately no dreadnoughts or decimators in the respawn. 

Got it down to 3 enemies remaining, of course, they were on the opposite side of the map. By the time I got to them several rooms respawned and I was back up to 25% or 30%.

If this is not a bug, it was rather disheartening to have the mission so close to completion, only to have the goal line moved.