I suppose this statement of yours is actually what made the other member confused since you are not correct. The situation you have described is not going to happen. If a stereo-only reverb is inserted on a mono bus then the resulting signal - before entering the stereo masterbus - will be still in mono so you cannot hear the left channel processed and the right one not.
The situation you have described actually happens in the opposite case: when you have a mono-only plugin which has been inserted on a stereo bus.
Anyway, there’s no need to argue here.
Best regards
Miloslav