We didn't implement Vivillon correctly and there are numerous TODOs throughout the sim's code marking that the Icy Snow form should be the base form. We have acknowledged it was an error in hindsight but like Marty already said, by the time we discovered an error was made it was not worth it to jump to fixing it because it was already wrong for quite some time. I still intend on eventually correcting this error, but the amount of cooperation that is entailed throughout the sim and client makes this a giant hassle that I don't have the time to tackle right now.
Icy Snow is
quite literally the base form in XY's internal code (as well as ORAS, SM, USM, and SV's).
The only wrong thing you're doing here is making Family of Three the default.
Making Family of Three the base form is correct and intentional, even if it's the rarer of the two forms in-game.
This has been spelled out for you four times now, so I'm locking this thread because we just aren't doing it lol