I don't think it's specifically related to FastAPI Users. When running behind a proxy, there are a few things to take into account.

I'll suggest you to take a look at Uvicorn documentation: https://www.uvicorn.org/deployment/#running-behind-nginx

Regarding proxy root, I think you should set the --root-path setting on Uvicorn: https://www.uvicorn.org/settings/#http

Indeed, I don't think this has anything to do with FastAPI Users in particular either.

I have no problems using FastAPI Users with Nginx.

I don't have a deep understanding of how Nginx and Uvciron work and how they are set up, so I don't know everything about your Nginx setup.

I don't know much about Nginx usage, and I don't know if you know much about Nginx.

But when I use Nginx, the front-end and back-end locations are separate.

I see that your location is written the same and I don't know if there is any problem.

You can try it first, create a simple API and setup only the backend in Nginx.

Or you can try to see where to modify FastAPI.

For example your API is

/api-service/fastapi/jwt/login
/api-service/fastapi/register
/api-service/fastapi/any-api

location /api-service/fastapi {
    ...
    proxy_pass http://localhost:8090;
    ...
}

location /api-service {
    ...
    index index.html index.htm;
    ...
}