This happened in survival and creative mode.
Im currently working on a mini game and i found something strange. i went to put a sign on a nether brick fence and it didnt work. the sign placed just fine but it didnt let me write on it. i then tried it with normal wood fences but that didnt work either.
i then tried putting it on the side of the fence instead of the top and the same thing happened.

Steps to Reproduce:
1. Get a sign
2. get a fence and a nether fence and place them on the ground
3. try putting a sign on the tops, and all other angles on the fences