item frames fall off wall when placed above fences

I placed a fence underneath an item frame (placed on a sandstone wall) as decoration. I expected the item frame to stay in place.

The item frame flew off the wall after roughly three seconds.
When I tried to place another item frame, it would not place one there.
I removed the fence, and then the process repeated

Steps to Reproduce:
1. place two sandstone blocks above each other
2. place an item frame on the top block
3. place a fence on the block below