@dancy was looking into why /favicon.ico requests aren't working on k8s. They are meant to be rewritten to /w/favicon.php, and indeed are, but there the trail goes cold. The http request that the MW PHP code makes from there is failing.
We don't yet have Logstash telemetry on errors here, but I suspected this might mean HTTP calls between wikis aren't working more generally, and indeed that appears to be the case.
The most prominent example, afaik, of HTTP calls within MW is File description pages which make API calls to Commons. For example:
When viewed over XWD/k8s, the file description is missing. This is potentially a cache poisoining issue as well since file descriptions are kept in memcached, and with a fairly higih TTL.