The wither storm needs invulnerability when it is born, because the tractor beam is developing when at phase 2 and the tractor beam is not ready to activate, and I also seen this video (sent as clip).
https://youtube.com/watch?v=NU4cPCBVL4o&feature=share&utm_source=EKLEiJECCKjOmKnC5IiRIQ&t=78

Don't think this would even be noticeable

I mean it would be cool for there to be a phase 1.5 when the tractor beam is developing

Don't feel this is necessary since you can't hurt the wither storm substantially anyways.

ok