Since upgrading to Symfony 3.4.0-BETA1 fom 3.3.10, suddenly all responses started receiving Cache-Control header with private, max-age=10800, no-cache, private value and a response code 304 Not Modified, which means that responses are cached in browsers for 3 hours and only hard reload with Ctrl+R or Shift+R refreshes the page from backend.

git bisect says that the culprit is #24523. And indeed, reverting the commit in that PR fixes the issue for me, meaning Cache-Control header is back to no-cache, private and the response is 200 OK.