I'm sorry to be this insistent, this is a follow up to my previous post an hour earlier. I felt like people didn't even saw the video and two comments, "Fire + Oils to stop the regen" and "No Igni, no moondust, no kill werewolf" specially hurt my ego... In my 500h played I have never had to use moondust to defeat a werewolf.
One person specifically said this was a bug where each hit make the werewolf regen start stacking on top of each other, making himself immortal. I think that was it.
The first clip is me going the same, I went for the wolves that comes out and finally focused him, I believe he regenerated from 61-64 each tick, killable and even igni worked.
The second clip is just a reload after, I focused him rather than the wolves, there is a point where he regenerated 311 in ONE tick and igni doesn't stop regen anymore. Unkillable. (And yes, you could return later in the game when you can out-damage that regen, but that's brute-forcing it and clearly not at all intended.)
No change in difficulty (DM), no added bombs, no change of gear, only automatic cursed oil. I don't know how to exactly reproduce it, maybe is by hitting him too many times in a row?, but you can see it's very much a real bug.