In a database with a textarea field, files can be embedded with the HTML editor. When calling the web service mod_data_update_entry to update an entry with embedded files, the files are deleted even passing the draftid as a parameter.

The WS ignores this parameter and instead uses a POST parameter sent by the form in Moodle web.

See https://github.com/moodle/moodle/blob/master/mod/data/field/textarea/field.class.php#L199, $draftitemid = file_get_submitted_draft_itemid('field_'. $this->field->id. '_itemid');