I’m using http://deb.debian.org/debian mirror, but I was surprised that it didn’t download files with apt-get update command. I removed /var/lib/apt/lists directory, and it worked and upgraded to 12.1 :).

I’ve upgraded packages several times. My base-files package has version 12.4. I’ve tried with apt update and apt upgrade -s and I don’t have any package to upgrade. Is it possible that the version is locked somewhere?