Once a divine nature incarnation is slain in the material plane in front of the public, news of this true god's demise will spread rapidly, causing many to transfer their faith, thus affecting the deity's true self.
Just like some evil gods, such as the famous Colored Dragon Mother, the five-headed dragon queen Tiamat, who has been slain multiple times throughout history, some of those times even by a group of newly minted Legendary heroes.
A group of ordinary Legendaries slaying the five-headed dragon queen Tiamat?
Don't joke.
Let alone the five-headed dragon queen; even one of her Legendary mates could take care of a group of ordinary Legendaries, not to mention her true self.
What they defeated was not the divine nature incarnation of the five-headed dragon queen, but a puppet secretly manipulated or a divine nature incarnation that had yet to be successfully summoned. As for the true self, it's likely still trapped in the Dark Dragon Nest of Barrens Hell.
