Assalamu aliakum, This issue of praying behind someone of the other madhab is in my opinion especially delicate in the Hanafi madhab. This is because in the Hanafi madhab the prayer of the follower is linked to the prayer of the imam in every way. For example the follower cannot pray his fardh behind an imam who is praying his sunnah. There are many such examples. Such is not the case with other madhabs as far as I know. For example in the Shafi fiqh if someone is praying nafl a follower who wants to pray fardh can come and join him. It is because of this that I think that if the Imam's prayer is invalidated (according to the Hanafi follower), the follower's prayer would be invalidated as well.