Does excessive prolongation of the madd in the Imam's recitation of Al-Fatiha invalidate the prayer?

The madd (lengthening) at the end of a verse, if followed by a pause on the letter "nun," is called madd arid li-sukun (madd due to a temporary sukun). Its longest duration is estimated at six harakat (vowel lengths). Anything more than that is considered disliked stretching (tamṭīṭ makrūh), though it does not invalidate the prayer. It is recommended to recite in tartīl (slow, rhythmic recitation) allowing for the madd and līn (softness) letters to be pronounced fully, without stretching or excessive melodizing.