Is distorting a Qur'anic verse – in writing – by mistake and delusion, without being certain of its authenticity, considered an act of disbelief (kufr)?

If something from the Quran is written with doubt, without certainty or strong presumption of its authenticity, then it is legally forbidden. This is because reciting the Quran with doubt about what is being recited is impermissible, and writing the Quran takes the same ruling as reciting it in this regard. Nevertheless, writing the Quran with doubt about its authenticity is not an act of disbelief (kufr), but rather a sin that necessitates repentance and seeking forgiveness.