Help the basketball ref with the balk rule

[Kw parent]

I was wondering if I could get feedback on a different scenario.

My son is a RHP.

With a runner on 2b he works out of the stretch. While engaged with the rubber he lifts his non-pivot leg in the normal fashion. While his non-pivot leg is coming up he turns his head (only his head) to check the runner. Then he turns his head back towards HP and completes pitch to the plate.

There is no pause or interruption in his movement. He just looks to 2b while in his movement.

Opposing coaches complain it is a balk but umpires disagree.

I am wondering if coaches might be right. It is not an interruption of his regular movement but it is an alteration and it is deceptive because he can easily continue pivoting to 2b in one movement and attempt a pick-off throw.

Part 2 to this question comes from my son in the event the prevailing opinion is that it is a balk. That is, why is a slide step allowed because that is also an alteration of his normal movement.

Thanks in advance for any guidance.