What is the reason for forgetting the Noble Qur’an and losing one’s memorization of it after reciting Surat Al-Baqarah with the intention of ruqyah and healing?

If your memorization slips without any negligence on your part, there is no sin. You must return to continuous revision and increase its amount. Present the Quran to someone who has a stronger memorization than you to solidify your حفظ (memorization), while seeking Allah's help and increasing your supplications. You can also perform Ruqyah (Islamic exorcism) on yourself with beneficial incantations if there is envy.