Pinnacle bosses bypassing leech immunity

Certain pinnacle bosses are able to leech life despite having the bloodless passive node allocated.

While I do have a mirrored amulet that allows enemies be able to leech 2% of physical damage as life, the bloodless passive counters it just fine.

I've let normal monsters hit me for a couple of minutes (not dodged or blocked) and they do not heal from damaging me, but pinnacle bosses are able to do so, even with no map mods that could grant them any sort of life leech whatsoever (or life regeneration of any sort for the matter.)