when the phoenix is devoured by a creature, why does it leave an ash outside the belly of the beast?
In flavor, I would say this:
The otyugh/scarab/mutant snatches up the phoenix in its jaws, and kills it with a sharp blow. The meat of the prey, which provides the bonus, slides down the throat, but the ash, being the powder that it is, slips between gaps in the teeth, thus winding up on the ground.
I don't know, just my theory