Why was the word "البنون" (al-banūn - sons) mentioned with a fatḥah on the letter nūn in the verse "Wealth and sons are the adornment of the worldly life," while the word "المال" (al-māl - wealth) is in the nominative case?

"Al-Banun" (البنون) is the plural of "ibn" (ابن - son), and it is appended to the sound masculine plural (jama' mudhakkar salim). It is raised with a "waw" (و) as a substitute for the "dammah" (ضمّة - nominative case ending). Its "nun" (ن) is open because it is the "nun" of a plural. It is conjoined with a "waw" to the word "al-mal" (المال - wealth) that precedes it.