This is actually because the Wither has a mechanism similar to stiffness. This mechanism appears when the Wither enters the second stage. While in a stiff state, the Wither loses the ability to fly and cannot stop its charging behavior, which lasts about 4 seconds at a time.
When the Wither enters the second stage, it does not immediately switch to the AI for the second stage; it switches after completing its current behavior list. If the Wither is forced into the second stage while in a stalled state, it will enter a displacement state after landing and exploding. Based on my observation, displacement and charging are essentially the same behavior, just expressed differently at different stages.
Around version 1.21, the stiffness mechanism changed. It could no longer fix the Wither in place, but would instead let it be affected by gravity. If the Wither then entered the displacement state (i.e., charging), it would charge downward and only return to normal after 4 seconds.
St1437
This is actually because the Wither has a mechanism similar to stiffness. This mechanism appears when the Wither enters the second stage. While in a stiff state, the Wither loses the ability to fly and cannot stop its charging behavior, which lasts about 4 seconds at a time.
When the Wither enters the second stage, it does not immediately switch to the AI for the second stage; it switches after completing its current behavior list. If the Wither is forced into the second stage while in a stalled state, it will enter a displacement state after landing and exploding. Based on my observation, displacement and charging are essentially the same behavior, just expressed differently at different stages.
Around version 1.21, the stiffness mechanism changed. It could no longer fix the Wither in place, but would instead let it be affected by gravity. If the Wither then entered the displacement state (i.e., charging), it would charge downward and only return to normal after 4 seconds.