If I open an image using the Photos app from Explorer, the Previous and Next image arrows on the screen work fine, as do the keyboard arrow keys. But if I open an image using the Photos app from Directory Opus, the Previous and Next image arrows are greyed out and the keyboard arrows don't work either. I don't have this problem using Windows Photo Viewer.
Another thing that I noticed is that I don't have as many "Open With" options in Directory Opus as I do with Explorer. Maybe that's a clue to my problem.
That is the Windows Photos app choosing to do things differently depending on how it is launched, and there's no other documented way to launch it that we are aware of, as with a lot of the "Metro" side of Windows.
See my reply here for more detail.
Thanks for your reply, Leo.
I guess it means that there is no solution at this point in time. 