A thought...
The GET request for ":50100/irj/portal/" could have followed a redirect from the server being rewritten from "/" to ":50100/irj/portal/". The Location header per RFC2616 must contain an absolute URI, but maybe this app uses a relative URI of "/"? If it did, the string map command would rewrite the redirect to the URI you're seeing the client request.
ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt
14.30 Location
The Location response-header field is used to redirect the recipient
to a location other than the Request-URI for completion of the
request or identification of a new resource. For 201 (Created)
responses, the Location is that of the new resource which was created
by the request. For 3xx responses, the location SHOULD indicate the
server's preferred URI for automatic redirection to the resource.
The field value consists of a single absolute URI.
Aaron