Thank you, I understand now. That viewer isn't part of Explorer, it's the Microsoft Photos app.
Unfortunately, it is also a "Metro" application, which means everything about it is obfuscated and there is no direct way to run it.
I can't tell you why it doesn't let you browse to other files when you launch it via Opus. All we do is tell Windows to open the file, and Windows does the rest. Windows or the viewer then appear to do something different in Explorer and Opus.
This happens even if you launch it via the Open With > Choose Default Program, which is even more outside of Opus's control. (There, we just tell Windows to display the Open With dialog, and everything else that happens is done by Windows. We do not even know which program was selected as the program choice and launch are all handled by the OS without our involvement.
This looks like a defect in the Microsoft Photos app, since it does not always enumerate the other files in the same folder.