مَحْفُوظٌ فِي قُلُوبِنَا
It is preserved in our hearts.
A memorizer’s retention, recollection and mental arrangement of words are human acts and states. They may arise, fade or change. None of this requires divine speech itself to move physically into the human mind.
The act of memorizing and what is memorized must be distinguished. A person learns, forgets or repeats a verse; these changes belong to that person’s memory. They are not changes in the divine attribute.