What is the ruling on praying behind an Imam who is suspected of committing polytheistic acts, such as writing amulets, and what should we do if it is impermissible to pray behind him and he does not attend regularly, and is it permissible in this situation to leave the mosque after the Adhan to search for another mosque?

The default assumption for a Muslim is soundness, and it is not permissible to harbor ill-will towards them, for Allah the Almighty says: {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} (Quran 49:12), and for the saying of the Prophet (peace and blessings of Allah be upon him): "Beware of suspicion, for suspicion is the falsest of speech." Therefore, it is not permissible to suspect the Imam of evil without clear proof, and prayer behind him is valid.

However, if his commission of major shirk (polytheism) is proven, then do not pray behind him, and report his matter to the competent authorities for his replacement. You may leave the mosque and go to another one. If what he does is minor shirk, then your prayer behind him is permissible and valid in all cases, as long as he does not become an infidel.