Is forgetting memorized parts of the Qur’an, then recalling them through quick review, considered poor memorization that incurs sin?

Preserving what one vowed to preserve is considered fulfilling the vow, and one's responsibility is absolved thereby, with the necessity of maintaining it, caring for it, and reciting it daily. For memorizing the Quran is a great blessing that requires gratitude and continuous revision, because it is more prone to escape than a hobbled camel. The inability to recite from memory except after revision is natural for someone whose memorization was not firm from the beginning.