The bug
Certain blocks above wither skeletons prevents them from pathfinding to a target they can try to reach if different blocks are above them.
Check out this relatively short, unedited clip of me trying different blocks above them: https://youtu.be/2vEF5HfJOLA
Linked issues
Comments

I can confirm for 20w46a.

See MC-101136; the wither skeletons' behaviour depends on whether they are considered to be in direct sunlight. To address the blocks used in your video: Lava, soul sand, farmland, dirt and enchantment tables all block sunlight from affecting mobs, while glass, trapdoors and chests do not.
I can confirm for 1.16.4 pre-release 1