We're currently on version 4.5.5.006 and I believe it's been happening for quite awhile. I'm not certain when it first started. I know the session clustering worked with Memcached when we initially deployed the application on Railo in 2014.
I've also tested with the Redis extension and it didn't make a difference.
In App CFC, we're setting this.sessionCluster = true.
If the user stays on the same server, the session is persisted, however, as soon as you hit a different backend, the session is lost.