Hmm, that's strange, I have always activated the option “Redefine Scaling as 100% for all text frames”. I'm only now realizing that the text flow issue I described above only comes to light when I copy the rescaled frames into a new document. However, the spacing of the text shadow, if it is grouped, already appears incorrectly in the scaled document, i.e. not only after copying into a new document.
To be more specific about the text outflow: visually everything looks correctly scaled after scaling, but when I select the area, it still says 30mm. As soon as I copy the frames into a new document, the outflow is effectively 30mm (and also visible)!
However, if I perform the scaling WITHOUT the option “Redefine Scaling as 100% ...”, ALL shadow distances of the text boxes are NOT scaled (regardless of whether they are grouped or not)!
However, the frame with text outflow behaves in exactly the same way, regardless of whether the “Redefine Scaling as 100% ...” option is activated or not.