Why does stopping at الْيَقِينِ allow temporary-sukūn madd?

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

• The lām becomes doubled
• Bāʾ follows yāʾ
• Rāʾ has tanwīn
• Final nūn becomes sākin after madd yāʾ