I was watching some of the games, but didn't see this play.
I don't work Little League games, so this is speculation on my part. When the ball "passed F3", how close was the ball to the fielder? I do know that some of their softball rules are based on baseball interpretations. Maybe they use the baseball definition of a ball "passing an infielder".
The OBR (official baseball rules) interpretation of a batted ball passing an infielder is that the ball must go "through, or immediately by" the fielder. In other words, it would have to go through the fielder's legs, or pass by within reach of being fielded.
Most softball rule sets (ASA or high school, for example) are a little more liberal in defining "past an infielder". If you draw a line between the two infielders, and the ball goes past that line, it is considered to be past an infielder, even if the ball is well out of reach from being fielded.
Again, didn't see the play and am just taking a guess at why they would call this interference, when most softball rule sets would not.