Does Fajr prayer and the prayer of those led by the Imam become invalidated if the Imam suspects the presence of a slight trace of stool resulting from a previous release of gas, and then this becomes clear to him after the prayer?

The fatwa (definition) we adhere to is that avoiding ritual impurity (najasa) is conditional upon knowledge and ability. Therefore, your prayer is valid due to your ignorance of the impurity's presence. Nothing is required of you for the doubt mentioned, because the default is the absence of impurity. If your prayer is valid, then there is no problem with the validity of the prayer of those who prayed behind you.