What is the ruling on the prayer of one who prayed 'Isha behind an Imam who prolongs the pause on the letters Meem and Noon at the end of the verses of Al-Fatiha, as if he pauses for a long time on the Noon in "Al-Alameen" to the extent of doubt whether he is pausing on it or pronouncing it with a shaddah?

It is clear that the questioner is afflicted with waswasah (whispers/obsessive thoughts), and he must not succumb to it, as it leads to great evil. The default is that the Imam's recitation is valid. What was mentioned is not a clear grammatical error (laḥn jalī) that invalidates the prayer. Most likely, the matter is merely an illusion and waswasah.