Viewport bug in XD 6

I'm prototyping a mobile project. After last update I get a huge black area both on each side of my layout in preview mode and on the top and bottom. When I change the wievport on the first screen in my layout it doesn't occur on that screen, but it occurs on the NEXT screen I linked it to without that being altered in terms of viewport at all. If I remove the pre-formatted wievport handle then that screen becomes small and it is surrounded by black, It seems totally out of control now. And where is that newly introduced wievport handle when you drag it all the way down? It just vanishes and is never to be found again. I beg you to go back to how it was! W10.