I can report that I've succeeded in reproducing the problem easily on OSX.
After reproducing the problem - with one diagonal line above the other - I also noticed some clipping errors where the group boundary cut out a bit of the top off the top line and a bit of the bottom off the bottom line. See attached screenshot.
Also, after resizing the group vertically larger, then smaller, the line width in "Properties" doesn't reflect the displayed width - it seems to be updated randomly with very odd effects. (I realise this isn't a very scientific description.)
Essentially line widths aren't being kept in line (excuse the pun) between what's on screen and what's shown in "Properties". However, there is an argument about whether line widths should change with resizing. Some would say yes, and give good examples of why, and others would say no, and give equally good examples. There is no right or wrong way to do this; it depends on what you're doing at the time and Scribus has no way of knowing what you want. The real problem is that you have no way of telling it.
I'd suggest reporting this as a bug - and related feature request - on Mantis but registration isn't possible at the time of writing (and I've probably got more than enough open issues under my name already!).