Changing the default to the Photos app reproduced the same issue on my machine.
From searching the web, this is a widespread bug in Windows 10. The Open With > Set Default option is simply broken right now. The quality control on Windows 10 is completely in the toilet these days. It's ridiculous.
This worked for me:
Open an administrator command prompt, then run:
assoc .png
That should report pngfile as the file type class, usually.
Assuming that's the case, then run:
ftype pngfile
It'll report the current association. (Not that important, but may be worth knowing before we change it.)
Then run:
ftype pngfile="C:\Program Files\GPSoftware\Directory Opus\d8viewer.exe" %1
If you get an access denied error, make sure you're using an admin command prompt and try agan.
Running ftype pngfile again should reflect the change.
Now, you'd think that would be enough to make things work, but it isn't. You'll still get the other program when you double-click. BUT, if you now use Open With to set the default to d8viewer.exe, it will work.
Windows 10 is shoddy piece of work. I'm starting to dislike it as much as Windows 8, albeit for very different reasons. Sigh.