Unless I've missed a patch note somewhere, I think they are indeed just officers equipped with shields - I forget which kind, lore-wise. Are they the bald (I think) dudes with a plasma pistol? Those are definitely officers, and always have shields.
The best bet is just to apply more firepower - in my experience nothing seems especially good or bad against them and raw power is the key, though I keep forgetting to use the shield-breaker shells so maybe their damage type is a clue. A single hit from an AT weapon usually does it, but bolters do OK and mass lasgun fire works in a pinch. The shields seem to regenerate pretty quickly, so once you've popped one you'll want to do everything possible to kill the officer that turn or you might have to do it all over again.
(Also, uh, get used to killing shielded enemies)
