Excuse me /vp/, but I got a question about something strange that I am hoping you could answer.

I had this shiny Gible that evolved into a Gabite and eventually Garchomp. Everything was fine until it evolved into a Garchomp, at which point, for some reasons I don't understand, it didn't have its shiny colors. Why is it just a normal Garchomp?

I was confused, so I tested with mega evolution, and it actually had shiny colors during its mega form. But then it reverted back to normal colors once the battle was over. Is this a bug I've never heard of? How do I fix it? Am I fucked?