I am not sure what rule you think that is breaking, but it can happen for various reasons if the heavy cavalry consider themselves disadvantaged - e.g. they are flanked or disordered etc. I wouldn't recommend charging heavy cavalry with light horse as a viable tactic - they won't usually evade ...

As a practical matter what troop types does this involve? In the TT game light troops must evade from heavies, and may evade from other lights. Certain cavalry may evade. (If I recall that was non-impact cavalry). So the player decision was fairly limited but having said that it could be a crucial ...