How to modify field values sent in notification emails

Field value formatting is controlled by ACF and should reflect the Return Format setting on a field. If you need to display a field differently in an email notification, the best way is to create a custom merge tag and use that in the email notification content field.

Consider the following example. A radio button field type has been used and its Return Format is set to Both (Array). As a result, in the notification email, the submitted value appears in the format value, Label. If we want to return just the label, we can use a custom merge tag for that field as follows:

add_filter( 'af/merge_tags/resolve', function ( $output, $tag ) {
  if ( 'YOUR_CUSTOM_MERGE_TAG' != $tag ) {
    return $output;
  }

  // Assume this field has a `return_format` set to `array`.
  // This config results in both the value and label appearing in notification emails.
  $value = af_get_field( 'RADIO_FIELD_NAME' );

  // Return just the label.
  return $value['label'];
}, 10, 2 );