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.
Thank you for your report!
We're actually already tracking this issue in MC-2349, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved. That means that this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki