I encountered a path matching issue while configuring Nginx to internally forward requests. When the path /api does not have a trailing slash, the subrequest is successfully handled by the verify controller and returns a response, but the success controller does not receive any requests, resulting in a 404 error in the browser. Adding a trailing slash /api/ resolves the issue. Here is the relevant configuration:

server {
    listen       80;
    server_name  localhost;

    location /api {
        auth_request /auth;
        proxy_pass http://localhost:8080/success;
    }

    location = /auth {
        internal;
        set $query '';
        if ($request_uri ~* "[^?]+?(.*)$") {
            set $query $1;
        }
        proxy_pass http://localhost:8080/verify?$query;
        proxy_pass_request_body off;
        proxy_set_header Content-Type "";
    }
}

The backend controller is running correctly and can handle requests:

@RestController
@SpringBootApplication
public class Application {

    @GetMapping("/verify")
    public ResponseEntity<String> verify(String token) {
        // Verification logic
    }

    @GetMapping("/success")
    public String success() {
        return "success";
    }
}

Question:
Why does adding a trailing slash / to the path /api affect the path matching? Is this behavior based on general Nginx syntax rules, or is it a specific quirk of the ngx_http_auth_request_module?

Thank you for your help!