Ultimate fallback action for TwoStageFallbackPolicy

I’m trying to implement a live agent handoff using the TwoStageFallbackPolicy.

The documentation for Fallback Actions says that the ultimate fallback action is specified by fallback_nlu_action_name. So I have specified it in config.yml:

- name: "TwoStageFallbackPolicy"
    nlu_threshold: 0.5
    ambiguity_threshold: 0.2
    core_threshold: 0.5
    fallback_core_action_name: "action_default_fallback"
    fallback_nlu_action_name: "action_operator_handoff"

But my action_operator_handoff is never triggered - after two stages of affirmation, the default fallback message is uttered.

How do I fix this so that, after the affirmations, the action_operator_handoff is triggered?