Why is the first nūn in عَنِ النَّعِيمِ not treated as a sākin nūn for ikhfāʾ?

Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf

• It has shaddah
• It carries kasrah in connection
• It meets bāʾ
• It has disappeared from the word