The bug

Item frames with the 'Fixed' tag set to true can be placed inside of other blocks, and do not break.

Steps to Reproduce:

1. Get an item frame with the proper tag(s) applied:

   /give @p minecraft:item_frame[minecraft:entity_data={Fixed:1b,id:'minecraft:item_frame'}] 1
   

2. Place down an end rod with a glass block in front of it (these blocks are not necessray to reproduce the issue, they just make it clear/easy)
3. Place the item frame down on the side of the end rod which faces towards the glass block
   → The item frame is placed inside the glass block
   → The item frame would not be placed as there is a block in the way

Screenshot: