When an item frame is placed in a horizontal direction, comparator output will only work in one cardinal direction (west when on the floor, east when on the ceiling).
Comparator output for item frames on the floor / ceiling should work from all directions.
How to reproduce:
- Place an item frame on the floor or ceiling.
- Place any item inside of it.
- Place 4 comparators on the same level as the item frame on each cardinal direction.
The four comparators read from the item frame and give a redstone signal output.
Only one of the comparators gives an output. For item frames placed on the floor, it is the west side. For item frames placed on the ceiling, it is the east side.
I assume this is a side effect of floor and ceiling item frames having the same properties as item frames placed on the east and west side of blocks, but differing visually and to which block they need to be attached.