Umm Abdillah mentions a narration from Ad-Dahaak which is narrated by Abu Ubayd in Fadaailul Qur’an with a hassan chain,
“There is not anyone who learns the Qur’an, then forgets it, except due to a sin he committed because Allah says, ‘
و ما أصابكم من مصيبة فبما كسبت أيديكم
And what ever of misfortune befalls you it is because of what your hands have earned” [Surah ash Shurah 30]
And indeed forgetting the Qur’an is from the greatest of misfortunes” End translation.
Majid Jawed Al-Afghanee