Sorry for asking but am I talking to an AI here? Isn't the video more than clear on this? Not to mention it should be reproduceable in all of 30 seconds.
I used explosives to drop my health to show the issue. Then you can clearly see the HP doesn't refill from hitting monsters until the point I let them hit me, in which case the next hit instantly refills the bar. It ostensibly doesn't count as being "in combat" with them until you get hit by them. It wouldn't matter where the damage came from (unless you're telling me your code saves that and somehow uses it in the combat medic calculation, but even so, I checked and explosives are unrelated).
I don't know your codebase, but I would assume that the perk checks a flag for whether you're "in combat with an enemy" or not, and that said flag isn't set until the enemy touches you. Not even sure anything else in the game uses that flag, but if it does, I would check those too. I always thought mastery gain from moster killing was slow, so that's one I would swing by the codebase of.
I'm not sure I get the confusion over such an ostensibly reproduceable issue, unless you've tried and cannot reproduce it. If so I'd make sure it's the same weapon, but at 1:07 in the video you can see the scythe also fails to heal. That would leave the chance of my save being permanently bugged with some flag that breaks it, but that seems very unlikely, your response has not indicated that (again, just seems like AI/copy paste responses), and the spaghetticode-implications of managing to create such a bug are somewhat scary.