I am not unfamiliar with mobs doing a bit of regen while running, but it seems mobs are regenerating several % per tick while in combat, like 3-5%, which is not normal unless you are Venril Sathir. This holds true even as you level, the regeneration rate stays proportionate.
Example- I just killed "A greater Skeleton" in Befallen, which has approximately 250hp. It regen'd at a rate of 4% per tick, which means he is regaining roughly 10hp per tick. That definitely feels out of the norm.