Looks more and more like a bug to me.
With all classes visible still no line. (nb my other viewports have lines and they are visible).
I notice the fact that the view port crop can be on two classes at the same time. In viewport crop edit mode it is on the class you created it in. In sheet view the view port can be moved to a new class without affecting the viewport crop edit class. I can edit the line width , type and colour in viewport crop edit. GREAT i wanted to do that.
I did an experiment. I created a a number of different viewport boxes on the design layer with a number of combination of off and on classes. Well the display of the lines is quite erratic. Seems to depend on a combination of factors. Sometimes you cannot edit them sometimes you can. Sometimes no line at all in the Sheet View. Going to have to give up on understanding what controls what here.
Any more tips on view ports? I have only ever been able to create one by drawing a box in the design layer and then using view>create viewport. The other method doesn't work.
BTW You cannot draw a border box (or any other line) while in Viewport edit crop mode. It won't let you.
David