No datasources with proxy auth

I have created my own proxy to handle authentication and everything is working well, however I have an issue with datasources.

When I create a new organisation I created a default (mysql) datasource with the API and everything works fine. The admin user can access the new datasources page but the dropdown list of datasource types is empty.

I see that browser calls http://host/grafana/api/plugins?enabled=1&type=datasource but the response contains an empty array.

Have I missed a step here?