I remember having a similar problem in 4.8 but it wasnt due to a Reparent and i cant remember how i fixed it, i think i recreated the classes like i suggested.
So your class hierarchy is now DefaultCombatant as the base class and DefaultCharacter inherits from that and then Mystic inherits from DefaultCharacter.
Try implementing the function as an Interface member, that will do the trick for sure, or if possible change the reference to the exact class type.