The render controller of wither had used the deprecated function over 6 years.

In the wither_boss.render_controllers.json here's a function called query.overlay_alpha which not working after 1.12.

I think mojang should use some new function for alpha channel like this:
      "overlay_color":

{         "r": "variable.is_invulnerable ? 1.0 : this",         "g": "variable.is_invulnerable ? 1.0 : this",         "b": "variable.is_invulnerable ? 1.0 : this",         "a": "variable.is_invulnerable ? Math.max(1.0 - query.anim_time * 0.2, 0) : this"       }

Use this function you can easily fix the animation problem.