Given a backup MX B using permit_mx_backup and permit_mx_backup_networks, which includes the IPv4 network of A, which it backs up: as soon as A's MX RR gets AAAA records, B's postfix rejects mail for A because it stops to checking the IPv4 address against permit_mx_backup_networks but only checks the IPv6 address. I think it should check both addresses and accept the mail if either matches.