"The auth window was closed" when using external authentication

We have a private oauth2 provider that I have set up for external authentication from Forge. The oauth flow authorizes and the token is exchanged, however the Forge app still shows “the auth window was closed” as seen in the attached screenshot. If I refresh the page, and hence the Forge app, the credentials are valid and the requests go through to the external provider, so I know the entire oauth flow is working fine. What could be causing this error message on the Forge side? (Note - it shows immediately upon the initial authorize request, even before the user has logged in or given consent).

Thank you,