Xbox One
What you were expecting to happen, and what actually happened:
In the Lost Souls of Carmina event, 5 purchases of 6 pets results in 30 pets, or 1 pet fewer than what is needed to ascend it to mythic.
How often does this happen? When did it begin happening? This is in the Lost Souls of Carmina event, which started today.
Steps to make it happen again
Multiply 6 by 5.
I have heard from the devs about this after Dust Angel bug reported it in testing. There is supposed to be 1 copy available by another means coming. I don’t know if I’m meant to say this or not or why it wasn’t advertised. Guesses are a Adventure board reward, a daily login reward or a free calendar reward. But I know people are sat there wondering so figured I’d say and take the slap on the wrists if I wasn’t supposed to
Hopefully not a cash offer 
You’re the best, thanks for posting this as I’m sure it will put people more at ease that it is probably planned and not to be chalked up as just another thing neglected or partially implemented. My guess would be you’re correct about having it in the AB right on Halloween or over the weekend like they’ve done in the past so everyone gets at least one guaranteed even if they don’t have the gems to max it fully in the event shop.
There’s going to be a freebie copy for everyone 
Good to know. Thanks