Smogon Forums

Ooh, it's been a long time since then but I don't believe it tells you anything? IIRC, it doesn't actually notify you that you've got the Magearna at all - you have to go to the gift section and it's there. So you can't try to claim it and get denied - it's only given to you at full completion, and before that there's just nothing there.

Could be completely wrong on that, though!

Did you try going to the gift section before obtaining all the mythicals?