It is frustrating but I don't think it is a bug, at least in RTW. In RTW when this happened to me, I blamed myself for asking my men to do a foolish charge without regrouping. I was always very careful to regroup and attack. Sometimes holding Alt and attacking seemed to force all your men to attack.

This is a problem with my Zweihanders too. As soon as one man engages in a charge the rest stop running. Its very annoying, especially when you are flanking and only one or 2 guys actually charges into the defending unit.
Do you mean you try to run Zweihanders behind and then charge into the back of engaged enemy? If I think about that, it would look rather silly and a little unrealistic if a unit were to do that in a real battle. Imagine them running in formation around the enemy flank and then told to face the enemy and charge. They are more likely to keep facing the enemy and overturn a flank. Maybe thats why it doesn't work properly when you order them to do it.