That also seems like a flaw that is entirely arbitrary and makes no sense.

Imagine I have a spell that prevents people from setting themselves on fire. Other people can set them on fire, and you can set others on fire, but you can't set yourself on fire. How does the spell know who's setting who on fire? If it just dampens fire in the area, that makes sense. Somehow having the fire suppression know whether or not one particular case of fire is self inflicted and stopping only that seems really really dumb.

And having this ability appear with no foreshadowing when fighting someone who powers up by setting themselves on fire, but is weak to fire attacks from others is REALLY shady.