When thinking some more about when it change in behaviour happened, I realised
that the freeze started about 2 weeks ago, so it could very well be related to
that.
It looks like the "apt-config dump" interpreted /etc/apt/apt.conf correctly, but
in case the format of that file is relevant, here's the content of that file:
Apt {
Default-Release "testing";
Install-Recommends "0";
};
Aptitude {
Autoclean-After-Update "true";
};