The doubled nūn in بِأَنَّ has ghunnah by virtue of its shaddah, not because another letter follows a sākin nūn.
The rāʾ of «رَبَّكَ» has fatḥah, so it is heavy. The doubled bāʾ must be articulated firmly but does not become a nasal consonant.
Contrast:
بِأَنَّ — ghunnah of doubled nūn.
رَبَّكَ — heavy rāʾ and clear doubled bāʾ, without nūn/mīm-type ghunnah.
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Doubled nūn versus heavy rāʾ
Al-Qāriʿah and Az-Zalzalah — Comparative Tajwīd Practice
The doubled nūn in بِأَنَّ has ghunnah by virtue of its shaddah, not because another letter follows a sākin nūn.
The rāʾ of «رَبَّكَ» has fatḥah, so it is heavy. The doubled bāʾ must be articulated firmly but does not become a nasal consonant.
Contrast:
بِأَنَّ — ghunnah of doubled nūn.
رَبَّكَ — heavy rāʾ and clear doubled bāʾ, without nūn/mīm-type ghunnah.