Can't get self-hosted Supabase to work as an OIDC Provider in Open WebUI

[MindNudgeLab]

Hey all :)
Hoping someone here can spot what I'm doing wrong. I'm trying to set up my self-hosted Supabase instance to handle logins for OpenWebUI using OIDC, but I'm completely stuck.
The problem is that OpenWebUI can't find the OIDC discovery endpoint. When it (or I) try to access .../auth/v1/.well-known/openid-configuration, we just get a 404 Not Found.
What's really throwing me off is that I've set GOTRUE_OIDC_ENABLED: "true" in my docker-compose.yml for the auth service. I've even used docker exec to check the environment inside the running container, and the variable is definitely there. The GoTrue logs don't show any errors on startup, it just seems to silently not create the endpoint.
I feel like I'm just one magic environment variable away from getting this to work. Has anyone else successfully configured this?
Here's the full environment config I'm using for the supabase-auth service.
code
Yaml

In my supabase/docker/docker-compose.yml

...

auth:
# ...
environment:
GOTRUE_API_HOST: 0.0.0.0
GOTRUE_API_PORT: 9999
API_EXTERNAL_URL: "https://[my-public-ngrok-url]"

  GOTRUE_DB_DRIVER: postgres
  GOTRUE_DB_DATABASE_URL: postgres://...

  GOTRUE_SITE_URL: "https://[my-public-ngrok-url]"
  # ... other standard vars ...

  # --- My OIDC Config ---
  GOTRUE_OIDC_ENABLED: "true"
  GOTRUE_OIDC_ISSUER_URL: "https://[my-public-ngrok-url]"
  GOTRUE_MAILER_EXTERNAL_HOSTS: "https://[my-public-ngrok-url],supabase-kong,localhost"

  GOTRUE_MAILER_AUTOCONFIRM: "true"
  # ...

Any ideas or advice would be hugely appreciated.
Thanks for taking a look.