npm's Proxy Setting Doesn't Route Everything Started by npm start
npm configures its own package transfers. The application launched by a script still needs an HTTP client that supports the intended outbound proxy.
npm install works through the proxy. npm start launches the app, and its API request goes directly. npm has not necessarily forgotten the setting.
The package manager made the first request. Your application makes the second. A proxy value in npm configuration is data for consumers that read it, not a tunnel placed underneath every child process.
Inspect the application's HTTP client and its supported proxy interface. It may read standard environment variables, require an explicit agent or dispatcher, or use another documented setting. Do not assume a variable resembling npm_config_proxy is recognized automatically.
Also separate outbound proxying from a development server's API forwarding feature. A setting that forwards browser requests from a development server to a backend has another role; it is not automatically the network exit for every request your application makes.
For a useful comparison, name the request that succeeds and the process making the request that fails. Then configure the latter deliberately. An application using Undici's fetch can follow its ProxyAgent dispatcher configuration, with a package release compatible with its Node runtime. Changing npm's registry endpoint or repeatedly reinstalling the application does not by itself supply that missing route.