Why did the scholars differ on the establishment of the Basmala at the beginning of every surah—except for Barā'ah—even though the Qur'an is definitively established?

The Muslim nation has unanimously agreed on the definitive authenticity of the Quran, and that whoever denies a single letter of it has committed an act of disbelief (kufr). However, they differed regarding the Basmala (Bismillah ar-Rahman ar-Rahim - In the name of Allah, the Most Gracious, the Most Merciful). Thus, whoever affirmed it or denied it was not deemed a disbeliever. The denier argued for its lack of continuous transmission (tawatur), while the affirmer sufficed with mere conjecture (zann). Imam An-Nawawi mentioned that the Basmala is Quran by way of ruling, and conjecture suffices for it. He also stated that the Basmala in Surat An-Naml is Quran by consensus, and whoever denies a letter from it has committed an act of disbelief by consensus.