code: protected function persistFlexForm(array &$valueArray)
      {
          foreach ($valueArray as $key => $value) {
              if ($key === 'el') {
                  foreach ($value as $idx => $v) {
                      if ($v && substr($idx, 0, 3) === 'ID-') {
                          $valueArray[$key][substr($idx, 3)] = $v;
                          unset($valueArray[$key][$idx]);
                      }
                  }
              } elseif (isset($valueArray[$key])) {
                  $this->persistFlexForm($valueArray[$key]);
              }
          }
      }
docs: Persists FlexForm items by removing 'ID-' in front of new
  items.
  
  @param array &$valueArray : by reference
  @return void