Browser responses with 502 containing a single 'date' header

Looks like HTTP/2, which only includes a header (request or response) if it's delta indicates a change from the prior request.

In other words, in HTTP/2, if the header is the same as the previous request it's not included again.

Obviously the Date header changes quite frequently, but most other headers do not.

If you are using proxy_pass, and talking to HTTP/1.x behind nginx, you'll need to configure your proxy_pass properly for your situation (Lots of answers on https://serverfault.com/questions/tagged/nginx with details on what you would need to do)

A 502 error is the NGINX proxy saying it received an incorrect response from the server.

You might want to do a manual 'get' with the same data as the call with the 502 error in Postman, to see if you can emulate the error.

If you cannot emulate it, the problem is most likely that the server cannot keep up with your requests, you can check if there are some optimisations you can do (client or server side), hundreds of request in general is not ideal (this could be a single request?).