yes, this is an expected behavior. The -F remove all files about the service. This means e.g. the /var/log/66/<service> directory and other. So, if your service is marked initialized the 66-disable refuse to remove all the files. You can remove e.g. the log directory from a service that can potentially be started.
So first stop it AND unsupervise it like this
# 66-stop -u <service>
Now you service will be not marked as initialized and so the disable command should works