Is it obligatory for someone who has forgotten a part of the Qur’an to stop memorizing new parts and immediately review everything they have forgotten, or is it permissible for them to combine both?

A Muslim upon whom Allah has bestowed the favor of memorizing the Qur’an should constantly review it, lest it slip away from him. Some scholars have even gone so far as to say that forgetting the Qur’an due to negligence is a sin and a major transgression. Whoever forgets it due to negligence must hasten to repent and review it. It is obligatory to memorize it again if possible, in order to rectify the sin. There is no harm in a Muslim dedicating a time to memorize new portions and another time to maintain what has been previously memorized.