Why does the postman respond intermittently, but the front is always cool?

Question

Nginx

we don’t understand why the postman hangs, more precisely, sometimes it hangs, sometimes it doesn’t.
and the site seems to be fine. requests are handled properly. all corses are allowed. in the nginx logs there are errors that port 3000 is busy and mongodb warnings, but the server itself takes 3000, restarting pm2 did not give anything