Hey! Not sure if there is a reason or not, but I noticed something different between the Birthstone Collectible Mugs. Some of them only have the option to adding it to Pet Treasures, some have Pet Treasures AND Other Options being "play with". Some are rarity 33 and some 50 but that made no difference.

Garnet Birthstone Collectible Mug Blue Zircon Birthstone Collectible Mug Diamond Birthstone Collectible Mug Sapphire Birthstone Collectible Mug

These four have have play and treasure option.