Is the Imam's recitation of "kaana" instead of "kaana" in the verse "Indeed, Allah was Forgiving, Merciful" considered a forbidden لحن (linguistic error), and what is the ruling on praying behind him?

This mispronunciation (laḥn) is very ugly, and intentionally doing it is forbidden (ḥarām). In fact, it may even lead one out of the fold of Islam if polytheism (shirk (definition)) is intended. The imam mentioned is ignorant and needs to be educated. He should be advised gently. Prayer behind him is valid because the type of mispronunciation that invalidates prayer is that which occurs in Al-Fatiḥah. As for the surah (definition) after Al-Fatiḥah, it is not a pillar of prayer, and an error in it does not invalidate the prayer.