Dragon bug in PVP All for One - Heals itself to FULL HEALTH from MID HEALTH while you are killing it

I was killing the dragon in PVP All for One mode, and had gotten it to half health, when all of a sudden it went to full health again while I am still chopping it. I think it's a bug as I have slain the dragon many many many times alone, and have never seen this happen.. please look into it.. Thanks in advance.