There is a bug in the new version of Classipress 3.1.7.
Checkbox values used to be stored in one WP_postmeta record, separated by coma. Now there are as many wp_postmeta records as checkboxes checked. The development team failed to update \wp-content\themes\classipress\includes\admin\write-panel.php file where such fields are exploded. Eg.: $options = explode( ',', $result->field_values ); line 253
It affects behavior of checkboxes, for instance fields with multiply checked boxes gets unchecked when the ad is edited/revised.