Express app becomes unreachable via pm2, works fine running directly

Providing a repro for this is sadly hard due to the Express app being huge, but the gist of the problem is that:

1. When running the app directly (node index.js), it works perfectly fine and responds to API requests in browser/postman.
2. When running via PM2, it becomes unreachable - it is still running correctly, with logs indicating no runtime errors. HOWEVER, all API requests terminate immediately in browser/postman with ECONNREFUSED state, and the app logs indicate no incoming requests whatsoever.

Where can we even start to debug this? Our only conclusion is that the error must occur somewhere in PM2 itself, but there are no useful errors or anything else being thrown.

Start by comparing the Express app launched with node index.js against the same app launched through PM2, using browser or Postman API requests and the PM2 application logs. The issue provides no reproducible project, PM2 command, or named test; done would mean identifying why the PM2-managed process refuses requests and documenting or fixing a reproducible cause.