ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_PREVIEW_TRACKING_REF_NOT_ALLOWED: A pull request preview tracks its pull request's branch - ngrok documentation

What does ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_PREVIEW_TRACKING_REF_NOT_ALLOWED mean?

ngrok returns ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_PREVIEW_TRACKING_REF_NOT_ALLOWED with this message:

A pull request preview tracks its pull request's branch. The 'tracking_ref' field cannot change.

Where can I get more help with ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_PREVIEW_TRACKING_REF_NOT_ALLOWED?

Email support@ngrok.com and include the error code.