I am not sure if this is an intentional feature or not but better to be safe than sorry.

While playing on a 1.15 snapshot server I made, I attempted to be fancy with designing and place two cobblestone walls attached to an entrance with a block of air between it because I know it doesn't have that attach-y model when you do that. When I placed a stone half-slab on the roof of the block, with there still being half a block of air, the item-place-y model appeared.

Again, I don't know if this is a bug or an intended feature, but if it is unintentional or something that needs to be updated then that would be nice.