The bug
I made a spawner that spawns item frames, and their rotation are random. l also made a painting spawner, but the paintings' rotation aren't random.
How to reproduce
/setblock ~ ~ ~ spawner{SpawnData:{id:"minecraft:item_frame"}}/setblock ~ ~ ~ spawner{SpawnData:{id:"minecraft:item_frame"}}
Confirmed this for Minecraft 1.12.1. The item frames seem to be also positioned randomly, and don't take into account where there's a block to attach to. They pop off after a second or so, like item frames always do.
I also noticed that you cannot put any item into the item frame.
This can be used by mapmakers as a method to generate random numbers, though.