Minecraft 17w18b, Minecraft 1.12 Pre-Release 1, Minecraft 1.12 Pre-Release 2, Minecraft 1.12 Pre-Release 3
In Pre-Release 3, melons and pumpkins are fixed, but fences and walls still connect to jack-o-lanterns.
Prior to the "things connect to the back of stairs" update, walls and fences did not connect to the sides of melons, pumpkins, or jack-o-lanterns. Now they do. This looks silly in fenced farms, and changes certain decorative details.
I'm not sure if this was an intended change, a works-as-intended side effect of "connect to solid blocks", or a bug. Since pumpkins and jack-o-lanterns already have special handling for their placement conditions, piston movement, etc, it seems reasonable to keep them as an exception for the fence and wall behavior.