The actual cause was my old custom admin theme. It was still referencing UI extensions (like the unmanagedAttributes tab) from the older version. Even though the backend provider was gone, the theme's JS and metadata were still triggering the frontend to request that route.

Fix: I completely removed the custom theme, restarted Keycloak, and the issue was resolved. The Admin UI no longer tried to load the missing route, and user detail pages worked normally again.