Happened to me when i did all brothers outside the tunnel, and only went down into tunnel for the last kill.
Kill sequence ahrims>karils>dharok>verac>torag>guthan.
I asked others about it, but it happened to them in a different order as well.
Also when someone else spawns a brother and you are on the same level, either tunnels or sarcophagus, the brother you're fighting will respawn.
It doesnt regen health or something its just the animation.
edit: i actually havent noticed much difference in hits with salve amulet equipped(together with ivandis), tho it could be that my hits were just bad.