What is the authenticity of the Hadith: "The sins of my Ummah were presented to me, and I did not see a sin greater than that of a man who memorized a verse from the Book of Allah and then forgot it"? And will a carrier of the Qur'an who acted upon it but then forgot most of it be forgiven?

The mentioned hadith is weak, and neglecting and forgetting the Qur’an is a great calamity and a sin. Some scholars have even considered it among the major sins. The warning in the hadiths and sayings of the predecessors includes those who forget a surah or an ayah, so what about one who forgets most of it? The Qur’an intercedes for its people who act upon it. It has been narrated in a hadith that the Qur’an and its people who acted upon it in this world will come on the Day of Judgment. It is possible for someone who forgets the Qur’an to still be acting upon it, and Allah’s bounty is vast.