The bug
The following blocks do not change walls into pillars when placed above, despite doing so in previous versions.
Cobwebs
Beds
Grindstones
For info, this issue was previously affecting the following blocks:
Linked issues
is duplicated by 5
relates to 2
Comments 5
Fence posts, iron bars, glass panes, flower pots, hoppers, sea pickles, end rods, signs, and walls are fixed in 20w15a. Pressure plates, cobwebs, banners, beds, and grindstones still do not update wall to pillars in 20w15a. Some of the remaining affected blocks might be WAI.
Iron bars already have a ticket, see MC-171463. Linking as related for now.