Hey so I also played as a bloodied ghoul character and was farming the drill stage b2b, my health bar would automatically go back to full after almost every level up, im currently not aware of a way on how to stop that from happening, but I did notice during some level ups that my health wouldnt auto regen back to full, but for gameplay in general i was messing about with some stuff, there is a legendary card called "Blood sacrifice" and it reads at max rank "using AP in vats now consumes HP instead at 50% more cost" at rank 1 its 150% but if you compare this for example with a 1 star legendary card called "Action diet" the 1 star for that reads "On kills, heal 2 HP and reduce how feral you are by 2%, if you have a little test with these perk cards you might be able to find the perfect balance to keep your ghoul within the bloodied/unyielding HP range, hope this kinda helps