What is the star particle that rises from my non-shiny Pokémon in battle?

Question

My Pokémon is not shiny, but every time I use it in battle, a star particle rises from it. Does anyone know what it is?

I think it's worth mentioning that he had been infected with Pokérus and is now immune.

Answer 1

Did you perhaps get a "Strange Ending" in Pokéstar Studios?

If the player manages to create a strange ending with their own Pokémon, that Pokémon will become a star and will be given a special entry animation, much like when a Shiny Pokémon is sent out
_{(Emphasis my own)}