Before the introduction of the 2020 Kubernetes ingress layer, the admin tool and/or the urlproxy front proxy provided some well-known urls like /favicon.ico and /robots.txt for the tools.wmflabs.org domain. The new setup allows tools to provide these themselves, which I think is a good thing, but it also currently does not step in to provide a reasonable default if the tool does not handle these routes.
We should figure out where we can intercept 404 responses for at least /favicon.ico and /robots.txt and return sensible defaults.