If X does not have his 'Buster' upgrade and you press 'R/L' to change weapons when charged or charging, you lose your charge state.
For Zero, this happens regardless.
(More than likely caused by an oversight. Zero should check if he has any sub-weapons and disable it if he does, otherwise continue charging. X's side, it should always charge no matter what.)
If X does not have his 'Buster' upgrade and you press 'R/L' to change weapons when charged or charging, you lose your charge state.
For Zero, this happens regardless.
(More than likely caused by an oversight. Zero should check if he has any sub-weapons and disable it if he does, otherwise continue charging. X's side, it should always charge no matter what.)