probe fails to detect out of date mirror

[Loïc Gomez]

Hello, user ven42 on #ubuntu-mirrors pointed out that http://ubuntu.mirrors.tds.net/ubuntu/ is very stale. Checking up on the mirror itself:

$ curl -fILs http://ubuntu.mirrors.tds.net/ubuntu/dists/focal-security/InRelease | grep -E "Date|Last"
Date: Wed, 08 Dec 2021 10:13:05 GMT
Last-Modified: Thu, 29 Jul 2021 09:32:00 GMT

$ curl -fLs http://ubuntu.mirrors.tds.net/ubuntu/dists/focal-security/InRelease |grep "Date"
Date: Thu, 29 Jul 2021 9:20:41 UTC

Probe's last log for this mirror is full of 404 errors, but it doesn't seem to affect its Up to date status (see attached, and https://launchpad.net/ubuntu/+mirror/ubuntu-mirrors-tds-net for latest log).