Mirrorlist is slow to catchup?

(or maybe my expectations are wrong)

So, now that download.tails.net is used in the Upgrader, I tried to use the mirrorbits interface instead of check-mirrors to look if download.tails.net found a working mirror to redirect me to. However now more than 12 hours have passed and:

• https://download.tails.net/tails/stable/iuk/v2/Tails_amd64_5.6_to_5.9.iuk?mirrorlist shows me that many mirrors have a working file
• https://download.tails.net/tails/stable/iuk/v2/Tails_amd64_5.8_to_5.9.iuk?mirrorlist shows me that no mirrors have a working file. It says "File size mismatch" for all of them. But that's wrong: I can download just fine from at least sv.mirrors.kernel.org, mirrors.wikimedia.org and archive.torproject.org

What's going on?