Alright, here you go. Uh...don't mind the 105 hours mark, haha.
I did some quick testing on multiple enemies with your latest save.
Killing enemy with single hit always regenerated you. Hitting enemies also regenerated you, no matter how little damage you did.
Only time when you didn't regenerate any health were when I changed your STR to negative value, which is understandable, since this is not supposed to happen under any circumstance and you would notice something else being wrong, since enemies would stop dying if that happened in your playthrough.
The other instance when you stopped regenerating were glancing hits.
These hits don't count as normal damage, so they don't trigger any regeneration and I don't think they even count towards your XP limit.
It is also possible that there is some enemy with broken elemental resistances and you keep "healing" him with ordinary attacks, in which case you will not regenerate anything.
If you notice a specific enemy that doesn't let you regenerate, please tell me.