From this thread (although I haven't encountered it myself)
http://www.adom.de/forums/showthread...Negative-Stats

High level chaos knights have stats fluctuating every step. For stats that affect HP (like strength), this means that their max HP changes every step. Since their current HP doesn't change when the max does (except to drop to a new, lower, max), any time the max goes up from the current HP they start healing. If they have the "Your wounds leave scars" corruption, this means they are regenerating far more often than other characters, even if they aren't actually taking damage, which means they receive a lot more scars than other characters.

Suggestions include adding/subtracting the same number of HP as the max changed (without letting them die when at low HP from this), keeping the same percentage of HP before/after, or some other mechanism.