When hitting an item in an item frame, the item will pop out and disappear usually after hitting another block. It usually disappears in the ceiling, sometimes being salvageable by breaking the ceiling block. If it disappears after hitting the floor, or an entity like a chest, there is no way to get it back (it wouldn't be in the chest either)

Noticed after losing my wither skull

I can post a video on Youtube if permission is granted. Or I can send a video file separately.