There is no evidence to suggest that the Prophet (saw) ever made du’a after the fard salaah in jamaah. Neither on the Sahaabah.
If one does make du’a by themselves once in a while after the fard salaah this is ok but if they do it repeatedly or believe it to be Sunnah, this could become a bid’ah.
Why would someone make du’a in questionable times when there are so many proven times to make du’a in?
This principle can be used for a lot of things.