Wither skeletons unaggressive/don't walk while in 2.5/3 block tall space

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).