Repeaters can be moved, so the row number may mean nothing if they alter the order of the repeater. They could also change the value of that row.
I would create an acf/save_post filter. I would use another field, outside of ACF to store “permanent_email_address” using a standard WP custom field using that used update_post_meta(), get_post_meta(), etc. When the post is first set up I would set this value.
