Forward Auth, update cookies not working

Hi Community,

I have a question regarding forward Auth. I`ve built a OIDC forward auth handler which verifies the token lifetime and shall request a new token if the old one is expired using the refresh token, up to here everything works fine. On the callback route creating the cookie works just fine but on the root route where I verify the token and refresh it if required updating the cookie, with the new tokens, doesn't work. As its easier to see the problem in code than me explaining it, here the code section in question:
Am I doing something wrong or isn't Traefik forwarding the set-cookie responses by design?
Thanks for your help!