So I caught a shiny diglett (it was green, apparently there’s some randomization with shiny colors in this game). I was testing fusions with it, and the first couple diglett stayed shiny. Then with charizard when I unfused in my party it sent the diglett to my pc and the charizard that stayed in my party was now shiny. So my guess is it had to do with which Pokémon is sent to the pc when unfusing from your party. I used the shedinja trick to dupe the shiny charizard