All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the situation is as you mentioned that the aforementioned Imaam pronounces the word “Iyyaaka” correctly, and sometimes you imagine that he did not pronounce the ‘ya’ with a double letter (yyaa), or that he did but lightly, then consider what you find as whispers. In this case, you should avoid these whispers and not give them any attention, and continue praying behind the mentioned Imam, and your Prayer is valid.
The basic principle is the validity of that Imam’s Prayer and that his recitation is free from mistakes that make the Prayer void.
Also, the basic principle is that your Prayer is valid, so it is not invalidated except with certainty. If you doubt that something that invalidates it has occurred, then do not pay any attention to it, and stick to the basic principle, which is validity, until the invalidity is proven with certainty.
We warn you against whispers (Waswasah), because it is a dangerous disease. So turn away from it, and do not pay any attention to it, because that is a beneficial treatment for it.
Allah knows best.