When I try to place an item frame above a fence block or a stone wall, nothing happens. There is a placing animation, but the space above the fence/wall is actively preventing it from happening. Item frames can still be placed on the sides and bottom of fences/stone walls, but not on the top or any space directly above it. This behavior does not apply if you try to place it on a ceiling above a fence/stone wall.

Video: https://www.youtube.com/watch?v=4A1mV3nngJw