Item Frames with the Fixed tag can be destroyed by hitting them with a projectile. 

Recreation:

1) Get an item frame and snowball from the creative menu

2) Place the item frame down on a block

3) Set the item frame to be Fixed:

/data merge entity @e[type=item_frame,limit=1,sort=nearest] {Fixed:1b}

4) Throw a snowball at the item frame

Expected Behavior:

The item frame should not be destroyed because Fixed item frames should not be destroyed under any circumstance.

Additional Notes:

If the item frame has an item in it, the item will pop off.

Setting the item frame to Invulnerable:1b also does not prevent it from being destroyed by a projectile.