Cors error when connecting through ssh tunnel

I often want to test my apis on a staging server. I use a **ssh tunnel** to connect with my frontend. Even if I for example allow `localhost:3000`, I still get cors error. If I use a reverse proxy with a domain etc. the same api connects to my local frontend without problems
Does anyone know how why this happens and how to solve this problem? Can it be a bug in the package?
Thank you