It's a display bug. It's not the only monster set that bugs that way. I remember playing PvP and seeing Scourge Harvester effect listed in the active effects with the same absurd duration, although the fight had ended long ago and the effect was actually removed - I was not actually losing health to another player. It's probably a format and/or math error. Instead of setting the displayed value to 0 decrements it to a negative number, and the sign bit is misread as first digit, subsequently decrementing a much larger value. Dying or re-logging the char fixes the error usually.