What is the ruling on someone who remembers the forgotten Fajr prayer due to a mistake in ablution, while making up other subsequent prayers?

Mentioning a missed Fajr prayer during the Asr prayer does not affect the validity of the Asr prayer, because the time for the Fajr prayer has already passed. The Malikis have stipulated that mentioning a missed prayer during a current prayer does not invalidate the current prayer, unlike mentioning the current prayer within the current prayer. Moreover, the order between prayers is a recommended act, not an obligatory one, according to many scholars. Therefore, your Asr prayer is valid, and you must make up the Fajr prayer afterwards.