Yet another balk question

[tankmjg24]

So I am just checking on my interpretation of the rule. FED rules if it matters.

In my game today we had R1 with LHP who picked his leg up and picked the runner off of first base. I am U1 and my partner who is in B position signals balk. From my vantage point everything looked good. Coach came out to my partner and almost as soon as he got there he went back to his dugout so no fireworks.

In our post game I mentioned that I did not see the balk and asked what the pitcher did. My partner said that while going to first that the pitchers free foot landed on the backside of the rubber. He did not cross back behind the rubber, just raised his leg and went to first and his free foot landed behind. Now I thought that this was a legal move as the pitcher is gaining ground towards the base he is throwing to and he is throwing directly to the base. I thought that the cross behind the rubber part of the balk rule came into play when the pitcher's free foot crossed behind then they attempted a pick off (other than second of course).

Thoughts?