Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Forgetting the Qur'an is forbidden. There is a consensus of scholars over this issue. The Shafi’e scholars even consider that a major sin. The evidence for this is the Hadith reported by Al-Tirmizi from Anas Ibn Malik that the Prophet (Sallallahu Alaihi wa Sallam) said: “The sins of my Ummah were exposed in front of me and I did not see any sin greater than the sin of a person who memorized a Surah of the Qur'an then forgets it."
Allah knows best.