Page cache is not detected but the server response time is OK

[djav1985]

New site health feature that reports on page cache says "Page cache is not detected but the server response time is OK"

Later expresses that"Page cache is detected by looking for an active page cache plugin as well as making three requests to the homepage and looking for one or more of the following HTTP client caching response headers:

cache-control, expires, age, last-modified, etag, x-cache-enabled, x-cache-disabled, x-srcache-store-status, x-srcache-fetch-status."

I do happen to have cache control expires age less modified and e-tags. But these are applied by nginx reverse proxy. Wondering why they're not detected.. When it checks the page is it running it by using localhost? Or does it try from the domain name? I think it should be set up to make a request to the actual domain name if it does not say that it would get a more accurate view of the headers.