There is no 0 index, repeaters start at 1 which is in the doc https://www.advancedcustomfields.com/resources/add_row/, when using. You would need to add a new row add_row() which will be added at the end and then update each row update_row() https://www.advancedcustomfields.com/resources/update_row/ with the value from the previous row and finally update the first row with the new value.
