No acknowledgement for successful account linking

Question

I'm trying to implement account linking for Google Conversational Actions, allowing users to connect their Google account to their account in our organisation's authentication system. Having read the documentation I decided that the best account linking type for our requirements is the basic OAuth linking type, and I'm implementing the Authorization Code Flow.

After several days of trial and error, I've implemented my own OAuth server and the account linking process appears to be working. However, when going through the linking process, after the final step is complete (i.e. Google calls my token exchange endpoint and receives an access token and refresh token), there is no feedback to the user, which is not a great experience.

The documentation here shows how to configure the account linking scene to handle successful account linking: "Configure how the flow should proceed if the user agrees to link their account. For example, call the webhook to process any custom business logic required and transition back to the originating scene."

I've configured my account linking scene accordingly. If I direct a user whose account is already linked to this scene, it successfully evaluates this condition and transitions to the next scene as expected. But when this scene is reached during the account linking process, this condition is never reached.

The process currently goes:

• User makes request that matches an intent which routes to a scene requiring a linked account.
• Google Assistant asks if the user is happy to be sent a sign-in link on their phone.
• The user agrees, and a link pops up on the user's phone, which takes them to my organisation's sign-in page.
• The user signs-in and is shown a page where they can agree to link their account with Google. If they agree...
• The web page closes.

And then nothing. No acknowledgement web page is displayed (as the documentation here implies might happen), and no voice prompts are played. In fact, at the point where the assistant says "I just sent you a sign-in link", in the simulator it says "{My test project} left the conversation".

Is this the expected behaviour, or is something wrong? It seems quite poor to me that after the linking process completes there is no acknowledgement. If this doesn't sound like the expected behaviour, what might I be doing wrong?

Update

I've spent many hours on this now but the behaviour is still exactly as described above. As an addendum, I've just noticed that not only do I get no kind of acknowledgement when account linking is completed successfully, but the Assistant doesn't use any of the customized prompts that I've entered when asking the user to link their account.

What I've configured:

What I actually see/hear when testing:

It's very frustrating that the account linking process is working but with a sub-standard user experience right now. Why is it ignoring the prompts I've entered? Why does it not do anything after a user successfully links their account?