FT has resolved the issue. The articles of the weekend sections are now displayed on
http://www.ft.com/intl/uk-edition and
http://www.ft.com/intl/international-edition. As of today, the Calibre recipe downloads everything correctly.
Unlike before the FT redesign, the weekend articles seem to be displayed even on weekdays. So your weekday issues may contain "Life & Arts" etc from last weekend. And the weekend issue may contain the weekday "Arts" section.
Anyways, I am happy now. Many thanks to everybody here for the help and support.