Origin rule not working.

I'm trying to remap the destination port in a request dependent upon a subdomain, specifically the final expression looks like

(http.host contains "foo.example.org")

(http.host contains "foo.example.org")

(http.host contains "foo.example.org")

(http.host contains "foo.example.org") however this is seemingly not matching/taking effect.

What could be the issue?