took me a while to figure out what exactly the bug was, but most players would have perceived it as UDK not actually dying even though he was taking hit after hit while at 1% HP.

The real bug is that his HP display is incorrect.  it shows 1% HP but he has more than that.  i believe this is a result of his "Didn't Need' Em" Passive where his HP% scales/increases when his allies die.  the increase in HP breaks the HP bar display.  

Example: my UDK is in Regen Gear and Immortal Gear, he should be healing for 18% of his HP every turn, but his HP bar was only going up by about 3 to 5% while all his Allies were dead.  so the HP bar was not displaying the true amount of HP he actually had left.