We are experiencing this exact problem using ColdFusion 11. I wrote a test page that makes 100 ajax requests at a given interval, alternating between a post and a get. You can reduce the interval setting in the java-script and eventually the ajax calls will start returning different session ids.
We are using latest CF11 running on windows 2012 R2 Standard with an IIS front end site. All current updates applied. Clustering and instancing have been set up per ColdFusion documentation to use session replication but not sticky sessions.
Any suggestions would be great!
i also have a similar situation, brand new install, 2 servers, 2 instances on each (cfusion and cluster_member)
a cluster set up on each server with the local member and the remote member.
session replication doesn't seem to work at all on CF11 with multiple servers, unfortunately i see nothing in any logs indicating any issues and obviously am not alone.
Vestige mentioned what first came to my mind: logs. Perform a test where you log te session cookies. Then the question is, are the newly generated sessions emerging from multiple instances in the cluster or from just one?