Main theme "Zen" is displayed instead of Subtheme "Zen Classic", if main theme is enabled too and subtheme is default

If the main theme "Zen" and the subtheme "Zen-Classic" are both enabled and the subtheme "Zen-Classic" is selected as the default theme, the main theme "Zen" is displayed instead of the subtheme. This problem also occurs with custom subthemes created by copying and modifying the STARTERTHEME. In the Administration theme setting the default is selected.

When checking with GARLAND and MINELLI, the problem didn't occur, so it seems to be a ZEN specific problem.

Is there any solution to this?

Thanks a lot,
Christian.