Why does the mīm rule differ in these two passages?

Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules

• Both have a following bāʾ
• Mīm always has one rule
• The next letter is yāʾ in the first and mīm in the second
• The first has nūn instead of mīm