Hi Andy,
Thank you for posting in Microsoft Community.
I see that you notice a feature change in 123 Photo Viewer app and I certainly understand it can be odd at times when you use the app frequently.
Let me help to point you in the right direction for this query. I would suggest you to check with the app publisher regarding the feature change as they would be the best point of contact to provide you details about the change.
You can check the app publisher contact details in Microsoft Store. Search for the app and scroll down to Overview section, where you should find the Contact Details.
You may also try to use the Photos App in Windows 10.
If you see all your images are blurry, let us know and we will certainly help you further.
Regards,
Vivian S