Another potential balk

[Nall23]

I'm looking for a little insight on a potential balk. It is not expressly answered in the rule book or already by this forum.

Last night I was field umpire for a pony league baseball game. The RH pitcher, when attempting a pickoff at first base, lifted his pivot foot, landed and wheeled to first. The potential problem is that his pivot foot did not clear nor land behind the rubber; instead the pivot foot landed ahead of the rubber, two feet to the right.

There are a few instances in the major league rule book that states a pitcher must step backward off the rubber 8.01 (b) and 8.05 (e) to become a fielder, but nothing in 8.05 states that a pitcher has to step directly behind rubber.

Is the fact that the pitcher did not step backward a balk? Would it have mattered if the foot had crossed the plane of the rubber and still landed in front of it?

Any thoughts would be appreciated. Thanks