Hi everyone,
I used to deal with this problem when trying to receive gifts from another account. And I see many cases also happen with bot and buyer in dispenser. Right after you receive the gift and choose store in inventory instead of adding directly to your library, you will see all of them vanished. Even when they are completely legit gifts.
This is a steam bug when the server busy. So to fix this issue, you only need to trade a trading card or background to account / or bot (for the seller) has the gifts vanished, then all of them will be shown.
Hope my little experience can help you solve the issue. GL :)