Answered: Loading multiple balls

At our last competition, a ref warned us for having 1 person loading 2 balls at once. I just looked thought the game manual, and I couldn’t find any rules against it. Was it a bad call? Or did I miss something? I’ve seen many videos of people loading 3 balls at once without getting warned.
I have no rules to quote because I can’t find any.

There are no rules prohibiting a Student Drive Team Member from loading more than one ball at once.

(In fact there’s nothing stopping a Student Drive Team Member from loading 32 Balls at once. However, this may prove difficult for most people.)