What is the ruling on praying behind an Imam who is suspected of innovation?

The default is that prayer is valid and not disliked behind imams, and prayer behind any of them is not ruled invalid based merely on suspicion. The default for a Muslim is to be free from innovations, and doubt based on conjecture is considered ill-judgment, for the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech." However, if there are significant indicators that elevate doubt to a strong probability, then there is no harm in acting upon their implications, as acting upon indicators is agreed upon among jurists.