Strange error from Channel Entry API: Submit New Entry

We are getting a strange error when submitting a new entry via submit_new_entry. While the new data is being checked (_check_for_data_errors) to make sure all required fields are present, essentially ExpressionEngine runs a database query to get all the fields, and if it’s required but no data is available, reports missing field value and the name label for the field

if ($row['field_required'] == 'y')
                {
                    if ($row['field_type'] == "file" AND isset($data['field_id_'.$row['field_id'].'_hidden']) AND $data['field_id_'.$row['field_id'].'_hidden'] == '')
                    {
                        $this->_set_error('custom_field_empty', $row['field_label']);
                        continue;
                    }

                    if (isset($data['field_id_'.$row['field_id']]) AND $data['field_id_'.$row['field_id']] == '')
                    {
                        $this->_set_error('custom_field_empty', $row['field_label']);
                        continue;
                    }
                }

Obviously SOMETHING is missing as we get these errors BUT the field_label is blank. In addition, as far as we can see the data is there. It feels like something strange is coming back from the query, but so many people use this I can’t believe it’s our own defect. The real kicker… we have 2 reports of this using iExpression out of 100’s of sites using the module. One using 2.1 and one using 2.1.1 both using completely standard field types (i.e. no 3rd party extensions).

Any ideas or clues?

Alan