UpdatePolicy in Autoscaling group not working correctly for AWS CloudFormation update

I am using AWS CloudFormation to launch my server stack. I have created a LaunchConfig and then AutoScaling group that uses the above launchconfig. I have set CreationPolicy which waits for signals from my EC2 instances which creating the CF stack.

Also, i have set UpdatePolicy to AutoScaling group to wait for the signals from new instances if i update the CF stack with more desired number of instances as follows:

"UpdatePolicy" : {
        "AutoScalingRollingUpdate" : {
            "PauseTime" : "PT10M",
            "WaitOnResourceSignals" : "true"
        }
    }

According to the above, CF should wait for signals from newly launched instances (or get timed out) before setting the status of CF stack as "UPDATE_COMPLE".

But it is not working as explained above. The status of CF stack immediately changes to "UPDATE_COMPLE" w/o waiting for signals.

Please help.