Not presented with login when accessing via reverse proxy after initial successful access

Setup external access via reverse proxy, got it working and everything worked.
Now when I go back to my home assistant via the external url it goes to a blank page.
For less than a second it initially shows the home assistant logo as if its about to log in then proceeds to this url and a blank page

https://ha.redacted.co/auth/authorize?response_type=code&redirect_uri=https%3A%2F%2Fha.redacted.co%2F%3Fauth_callback%3D1&client_id=https%3A%2F%2Fha.redacted.co%2F&state=eyJoYXNzVXJsIjoiaHR0cHM6Ly9oYS5tYWNhdmUuY28iLCJjbGllbnRJZCI6Imh0dHBzOi8vaGEubWFjYXZlLmNvLyJ9

I can still access my home assistant successfully on iOS externally via the hostname that will not work in the browser after accessing it once.

What’s it redirecting me to/for and how can I get it to behave as expected?
Perhaps, where have I gone wrong?