Why can't I get POST parameters?

Question

PHP

There is a small site on PHP 7.4 that spins in kubernets-e. How the server is nginx.
I take post parameters from php://input
Sometimes it happens .. that when a json request is made to my site from outside, it has this php://input empty, although the data is transmitted and because of this an error is returned that the POST is empty. It lasts for a couple of minutes or seconds and then everything works fine again.
I remember I had this with GET, but there I messed up with the nginx settings, and here is the magic.
Another important point is that the site itself does not suffer from this. This problem is only with json post requests from outside to my site.
Can you suggest what could be the reasons?