user engine session id re-generating on post

I get a redirect from user_engine when using

curl -i -X POST -d "<xml_data></xml_data>"
http://<webpage>?_session_id=ABC

If I use
curl http://<webpage>?_session_id=ABC

the GET works fine.If it was a routing issue, I would
expect the GET not to work.I also removed the -i, but no difference.

I'm using

if request.post?