This recently added result code indicates that Squid tried to refresh
the previously cached response A, but got a response B that was older
than A (as determined by the Date header field). Squid ignored response
B. Squid probably served the response A instead.
This "ignore older responses" logic complies with RFC 7234 section 4
requirement: a cache MUST use the most recent response (as determined by
the Date header field).
Frequent TCP_REFRESH_IGNORED entries may warrant an investigation. You
may be dealing with broken origin servers, unusual race conditions (that
Squid might be mishandling), and/or Squid bugs.