If both the input field and the repeater are shown then you cannot use add_row().
The reason for this is that with a priority of <10 ACF has not yet saved new values. When it does it saves the values that have been submitted in the repeater, overwriting the changes you made.
To add a row to the repeater you must alter the array in $POST['acf']['field_615acd90fb1e4'] to add the new row so that ACF can update the values.
