The bug
While Wither skeletons are standing in a 2.5-block-high space (or in a 3-block-high space while standing on a slab), they will not charge at you. However, if you get too close, they still hit you (and subsequently inflict you with the Wither effect).
Linked issues
is duplicated by 4
MC-121870
Wither Skeleton can't see player under a top half slab
Resolved
MC-136308
Wither skeleton does not move in 2.5 and 3 block tall areas
Resolved
MC-158475
Wither Skeletons cannot pathfind into areas with half slabs directly above them
Resolved
MC-177815
Wither Skeletons Can´t walk in 2,5 Block high spaces
Resolved
Comments 10
Isn't this MC-136308?
Confirmed for 1.13.2-pre2, but looks to be exactly the same issue as MC-136308. This ticket wasn't updated as much so if any ticket ends up getting closed as duplicate I think it should be this one.
Still present in 1.15.2 but wither skeletons can walk in 3 block high spaces just not 2,5 block high spaces
Relates to MC-87093.
Still present in 1.13.1; it seems as though the wither skeleton does recognize the player, but is unable to actually reach it.