I mean, they refer to it as an accident in the game, and the cutscene is practically identical save for the red lighting. I can’t find anywhere else (via google) that uses the term “fusion mutation” to refer to this as something distinct from a natural fusion accident. It also seems to follow the general rules of what makes a fusion accident — an unexpected fusion outcome that can be better or worse than what you have, and ignores level restrictions. The only real difference is that there are changes to the skills inherited, but that’s not limited to the yellow-named personas fused during an alarm. So it would make more sense to refer to the changes in skills as fusion mutations, and the actual personas produced when fusion fails as fusion accidents, no?