Hi Craig
I just noticed the app default change for PDF files from Adobe Acrobat Reader to Microsoft Edge - it happened when I was unzipping a program update I'd downloaded.
As WinZip extracted each part of the zip file, I was getting onscreen messages about apps being reset. I didn't take much notice to be honest, but I did see each warnings being added to the Windows notifications area on the taskbar.
Once the program was unzipped and installed into Photoshop, I checked out the warnings again.
As you can see specifically, the PDF file association was reset to Edge (from Adobe) in the process:
I don't know if this is what's been happening every time these default apps have been changed mate, but I've never seen a warning pop up like that before.
Seeing as I was using WinZip at the time, I've just tried resetting the default back to Adobe and then running Winzip again. This time I got no warnings, but crucially the PDF icons on my desktop changed back to Edge.
Further to this I've tried zipping and unzipping with WinZip on quite a few files, and every time the reset back to Edge occurs.
It would be interesting to see if you get the same results mate, and that we may have identified when this reset occurs, if not
why it happens.
Incidentally, using ZipGenius to zip/unzip does not cause the problem.