The dashlet URL should be saved in
civicrm_dashboard table in format eg.
When the dashlet is loaded it is passed through
CRM_Core_BAO_Dashboard::parseUrl() which converts it to the correct format for the CMS.
The dashlets were working on drupal7 because the URL format is similar but were causing an infinite redirect in Wordpress because they were being saved wordpress specific params (eg.
page=CiviCRM) which were added again.