Wither often retreats when he flies forward.
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This is likely related to the overall pathfinding behavior the game uses for most mobs. Often times, if an entity (a villager, for example) has a destination in mind and is moving towards that destination, often times that entity will not account for a sudden change in location, such as teleportation, and keep moving towards their destination, not stopping for a quite while until they arrive, or realize they can't reach it. Something similar might be happening with the Wither in this instance.
This issue affect 1.20.32