This is a bug/deficiency in Spring webflow. It is mentioned in Spring docs (see http://static.springsource.org/spring-webflow/docs/2.3.x/reference/html/ch13s10.html) but in a rather obscure fashion and does not mention relation to partial rendering issues. The fix is simple, add the following in webflow:flow-executor tag:
<webflow:flow-execution-attributes>
<webflow:redirect-in-same-state value="false"/>
</webflow:flow-execution-attributes>