While a floating Fence or Wall block has a texture underneath, if you place a bell under said fence/wall, the block's bottom texture will become missing. 

I think this occurs because the wall and fence think that the bell is a solid, opaque block, while in reality players can see past the bell and find that the wall/fence is missing a texture on the bottom. If the block above the bell is a fence, nothing will change if you connect it with another fence. However, if you do the same with a wall, the wall will regain it's bottom texture once it is connecting with another wall.

Another thing to note is that you cannot place a wall or fence directly above a bell. Instead you must right click on an adjacent surface to accomplish this.