@Guenter K Nothing to do with that. I can just create a new xojo project and only drop 1 groupbox on the window. As long the width is even, its ok, but whatever uneven width I chose (101, 201...) the right border vanish. Thats definitely not right.
You should report the bug through the feedback application, with all that evidence attached. You can download the latest version of Feedback from https://www.xojo.com/download/extras.php (log in required).
Until this is fixed, which may take some time, seems to me a workaround should be possible by making sure width is always even.
If GroupBox1.width mod 2 <> 0 then GroupBox1.width = GroupBox1.width -1
This in the resized event of the window should do it.