Wither AI having difficulties pathfinding to entities below blocks

The bug

The wither AI has a difficult time when trying to attack entites under a platform.

How to reproduce

1. Build setup shown here (wool blocks are for height indicator).
2. Summon a wither (/summon minecraft:wither) about 4-3 blocks above the platform side.
   → Wither can perfectly go down but the pathfinding has difficulties as shown here