In the current snapshot (20w06a) when a wall block has certain partial blocks placed on top of it, the wall does not change state into a pillar/stump that accommodates it.

This is something that I first noticed with string (which I often use to decoratively change the state of wall blocks) however for the sake of this report I dug a little further.

Objects found to not interact properly when placed on top of a wall include: Iron bars, glass panes, hoppers, pressure plates, sea pickles, cobwebs, fences, flower pots, end rods, and the soulfire lantern. 

Checking back in 1.15.2 all of these (where applicable) would interact with would cause a wall to turn it into a stump. Attached is a screenshot showing my test to prove this.

Also attached is a screenshot from 20w06a showing the two forms of lantern interacting differently with walls, as I think this is the clearest and most amusing example. other items I tested are shown in my hotbar, however I could well have missed something.

Thank you