Not a bug. Legality of certain shiny Pokémon were updated.When trying to play a doubles match, I got this message:
"Your team was rejected for the following reasons:
Diancie may not be shiny."
If there's some reasoning behind that then I'm curious to know it, but if not then I guess it's a bug. After some testing it actually lets shiny Mega Diancie go just fine, but if regular Diancie is set to shiny the error pops up.