Resizing constraints do not work when the width or height of the group is set via keyboard entry in the Styles panel. Constraints do work as expected when the group is resized by dragging the container handles with the mouse though. In this example, I have a horizontal line & a text rectangle grouped together.
I don’t know why constraints wouldn’t be followed when entering a value for the group’s height or width as it’s much easier to specify than mouse dragging to a specific pixel value. Screenshots below: