I dont see any issue with either "GET" or "POST" request when I use FMS as proxy and route these requests to any other HTTP server. I tried with the same settings as you mentioned above.
Can you let me know what FMS version are you using and also any of the request which is not being proxied I will try to simulate something similar to repro the problem.
We are using Flash Media Server Developlent Version 4.5, the operative system is Linux AMI ( from Amazon EC2 ).
Whe use a drupal site, this site works fine if no fms proxy is used. On the site is activated the blog module, the problem is we couldn't create a post, when I see the apache logs I didn't see any request in log, for that reason I think fms-server tooks as a streaming request.
We have similar problem, when FMS(port 80) is taken as proxy of Apache(port 8080).
When a POST from client to server, in >50%, no response from the server, and I captured the packets using wireshark, found that no 200OK from http server.
To figure out if it is the problem of Apache or FMS, we access the web form in port 8080 to Apache directly, the 200OK is sent back very immediately in 100%. So we can draw a conclusion that the problem is from FMS.
So FMS guys,could you figure out how to solve it???