What is the ruling on an Imam who connects verses to one another and does not pronounce the diacritical mark of the last letter, such as not pronouncing the dammah on the letter noon in (nastaʿīnu) in Surat Al-Fatiha? And is this considered a grave error (laḥn jalī) that invalidates the prayer?

What the questioner heard from the Imam is not considered a clear melodic error (laḥn jalī), so his prayer and the prayer of those behind him are valid. However, it is not permissible for the Imam to intentionally commit such an error, and he should be alerted. A melodic error that does not change the meaning is disliked (makrooh) and intentionally committing it is forbidden (haram), but it does not invalidate the prayer or the recitation according to the majority of scholars.