Google Assistant - Not handling auth callback?

I’ve setup my Google Assistant project, and attempting to get it into the GH app. I can see the test project and when I click on it, it asks me to login to HA.

I login successfully, it hands off to Google and then I can only assume that Google attempt to call the /api/google_assistant endpoint for HA.

What happens at this stage though is that the GH app just reverts back to the Works with Google integration list, and never seems to integrate HA.

I’m running this through duckdns, and my certificates are OK. The callback domain I’ve setup is https://<domain>.duckdns.org:<port>/api/google_assistant

Anyone know whats going on?