Suddenly immortal enemy

An essence monster with the gain endurance charge when hit and soul eater suddenly turned immortal at ~10% hp. The only other monsters around were spawned by the essence mods, had been fighting it for 10-15s and it suddenly stopped taking damage at all, recharged all ES and didn't take damage until after I had cleared the rest of the map and the map boss. The HP bar was moving down noticeably untill it suddenly stopped. I have resolute technique so couldn't have been missing etc. (Statistically impossible with number of hits I produce anyways)
Bug report reference number: 2618401962