[keycloak-user] Backchannel logout on session-timeout?

I'm using Keycloak 3.4.3 and have my java client configured for backchannel logout. It works fine if I logout from within the Keycloak user interface. But I was also expecting it to work if the KC user session died due to idle timeout or max session life. But in both session expiration cases, the backchannel logout is not triggered. Is this a bug in KC, or simply not a use-case that is supported? Or is this a configuration issue with my KC install? Thanks, Eric