ט:כ"ד וַיִּיקֶץ נֹחַ מִיֵּינוֹthe בעל קריאה mistakenly said וַיִּיקַץ as it is in the beginning of ויצא and מקץ.
I corrected him out of reflex but later wondered what exactly is the difference between the two. As I always ask: If there's a difference, what is the difference. If there is no difference, why are they different?
Once again, Dikdukian is saved by its knowledgeable readers. Yaakov Gross explains:
The later examples (accent on ultimate syllable, with פתח) are the normal form. The first example (accent on penult, final vowel סגול) is נסוג אחור, because it leads into נח whose first syllable is accented; that shift of accent causes a change in the final syllable’s vowel. (The shift in accent is very common with וי"ו ההיפוך, and that often leads to a change of vowels as well. A very familiar example is ויברך (in ויכולו): accent move to ב, and vowel of ר changed from צירי to סגול.