Since the last server stability reset at ~1:30AM EDT on 31Oct2015, monster HP has begun rubber banding. The monsters HP will drop to 0, then suddenly shoot back up to 50%.
Is this all monsters, or specific ones?
IIRC, there are some mobs that have an ability that lets them do this, but it wasn’t an every-mob thing.
Maybe lag? Some (all?) mobs do regen HP over time.