Preauth to RDG Gateway Using APM

Hi all. I am trying to assist someone who is trying to preauth users prior to reaching their RDG instance. At this point they do not want to use the BIG-IP as an RDG replacement so they would rather just auth using APM prior to allowing users access. When configuring the BIG-IP to use LTM only users are able to successfully connect the RDG and launch application successfully. When introducing APM, users can authenticate to Microsoft RDG, select an app and it immediately fails with the error that the server is temporarily unavailable. Has anyone run into this or successfully deployed APM in front of a Microsoft RDG farm?