The bug
Normally Creative mode players can damage and kill Invulnerable:1b
entities, however they currently cannot damage (or remove the item of) Fixed:1b
item frames.
How to reproduce
Switch to Creative mode
/summon item_frame ~ ~ ~ {Fixed:1b}
Left click the item frame to destroy it
❌ The item frame is not destroyed
Related issues
Comments

Appears to be fixed in 20w12a, possibly due to fix for MC-174258