So, what is your claster configuration ?
For a Tomcat 5.5 ( http://tomcat.apache.org/tomcat-5.5-doc/cluster-howto.html ), concurrent request from same session for a different claster nodes can override changes on other nodes.
Also, Tomcat claster service can have a different replication modes ( sync, async, pooled ).
So, there is recomendation for a claster front-end balancer : request for a same session must be served by one claster node.
It has been confirmed there is a cluster bug in jsf-ri 1.2_06.
Look for the soon to be released jsf-ri 1.2_07.