The wither Boss seems to have the wrong height value, Why i think that:

Nametags (using Addons to always spawn them with name) are on the ground

Riding the wither makes you appear at his 'feet'

When i change the scale, the particles of the effect are not 'resizing', so you have a small 0.2 scale wither but a 1 scale particle surrounding.

Making the Wither invisible does not make the charge animation invisible.

Also he is only rendered when the feet is visible.

I added some screenshots where you see the Nametag issue and the particles.