“The leg crossed the centre line” is not a complete explanation of a knee-reaping call. A referee evaluates the entire configuration, including control of the trapped leg and where pressure is being applied.
The decisive question is whether the configuration creates prohibited inward pressure on a trapped knee under the applicable division rules.
Do not rely on a social-media screenshot before a tournament. Check the current official rulebook, the age and belt restrictions, and any event update. When in doubt, ask the head referee before your division begins.