There is a solution to this issue which is why I moved your post to #ask-the-community. The way to back to portrait from multi view (without picking up your iPad) is to tap any camera to go to single-cam view. Then tap the box in the upper right corner that looks like a road intersection to rotate back to portrait. Then you can use the back button twice to get back to the camera/group list.
That said, I find this very annoying too. I keep my iPad in a stand which won’t even let it be put in portrait orientation and still be charging. Please hop over to the link below and vote for it. You need to click the VOTE button at the top for your vote to count: