If you open the Rumble Prize Box and your inventory is full, it places the toy on the ground (It was Maiev for me), but doesn’t put the actual toy in your inventory to add to your collection.
You seem to be able to earn new appearances for that toy after that, but have no toy with which to summon it. I also haven’t received it in the mail like what would normally happen in this kind of situation.
Edit: You also get credit for receiving that toy for the achievement, but do not actually have the toy anywhere.
You right click it and place it on the ground.
Normally this would place the toy on the ground and add a collectible toy to your inventory. But if your inventory is full, it places the toy on the ground, tells you your inventory is full, but doesn’t place the toy in your bags.
Update: As far as I can tell, there is no other way to earn the missing toy if this happens to you. You can seemingly get the “get a coin” quest on alts, but you can’t complete it if you have already gotten the coin on another character. Not sure how to get Maiev now that this bugged on me.