I think this is working as intended. When you are in soulsand you are "inside" this block causing to be able to touch the lava block.
Can recreate 1.16pre2.
Killed about 40 wither skelleton's, 40zombies and 40 normal skeletons. No rare drops at all. Using Looting 3, Smite 5 Sword.
[media]Easily recreated in 1.16pre2. Tested all neutral mobs, seems like only pigs have this problem.
Nevermind, I'm just dumb, I was thinking that you can smelt Stone in Blast Furnace.