Hey thats my head.

Could be one of a million reasons.

1. You’re lying.
2. Someone trusted in the claim removed it.
3. You placed it when the server was restarting and it never saved.
4. You never placed it, but thought you did.
5. A staff member removed it.
6. It wasn’t claimed.
7. As its stored in a item frame, its classed as an entity which isn’t the smartest way to store an item is it.
8. Someone figured out how to glitch items out of item frames (Highly unlikely and what you are suggesting)

Im going to say its one of those things.