OAuth Authentication not completing

Hello.

I followed this guide to set up OAuth in my Zendesk app to connect to my own back end.

https://developer.zendesk.com/documentation/apps/app-developer-guide/making-api-requests-from-a-zendesk-app/#managing-third-party-oauth-access-tokens

I have added this to my manifest.json

"parameters": [
  {
    "name": "token",
    "type": "oauth"
  }
],
  "oauth": {
    "client_id": "xxxyyyzzz",
    "client_secret": "wwwqqqrrr",
    "authorize_uri": "https://mybackend/oauth2/authorize/",
    "access_token_uri": "https://mybackend/oauth2/token/",
    "scope": "read write"
}

I built my app, packaged with `zcli apps:package` and uploaded to Zendesk.

Now when installing the app I see the link "Sign in with my app name".

I click this and it takes me through the login flow which completes successfully, and I see a blank page saying:

OAuth authentication has been completed successfully. You can close the current browser window and continue with the app installation page.

The URL of this page is:

https://<subdomain>.zendesk.com/support/apps/oauth/zis_token?app_id=xxxxx&verification_token=yyyy

I close the tab and go back to my Zendesk apps installation page, but I can't see the green check icon, and if I try to click on Install it says OAuth authentication is required.

What am I missing?

Thank you