With https://gerrit.wikimedia.org/r/#/c/408832/ we're sending public, max-age=86400, s-maxage=86400
This is not ideal, because there is no purpose to s-maxage if it's set the same as max-age.
Instead, we should be setting stale-while-revalidate so the cache can fetch a new tile in the background.
last-modified (always == epoch(0)) and etags (varies between nodes) headers are still wrong and should be fixed or removed (tracked in T187300).
This task is related to T108435