When playing in a realm, any item frames that have items in them become indestructible (making a popping sound when you try and break them), while ones with nothing in them can still be broken.
Works fine in single player, however.

Single Player: https://streamable.com/zo98e
Realm: https://streamable.com/thu3i 

The only way to break an item frame with something in it is to destroy the block behind it, whereupon it will hover in mid air for about five seconds then pop off.