I got this in an email from SU Support this afternoon,
Thanks for your note. You are absolutely right, and I’m told this is fixed in an upcoming release. I’m sorry you experienced this issue, and thank you for the feedback. As a workaround until that release, it helps to rotate the viewport model slightly, pick two dimension points on the same plane, and then use Scene selections in the context menu to snap the view back into Ortho view.
I don’t find the workaround very helpful, but maybe 2018 will have this fixed.