A customer reported that each time they restart their Windows 10 PC, the file association for PDF documents keeps getting reset to the default, which is Microsoft Edge. They use the "Set Default Programs" control panel to change the default handler to Program X, but the changes don't stick past a reboot.
The feature team for file associations explained that Windows 10 changed the way the user's file associations are recorded. Program X wants to set itself as the user's preferred handler, but they want to do so without requiring the user to confirm the change. How considerate of them. So they manipulate the registry keys directly. (I bet somebody got a really nice bonus for that feature.) But they are manipulating them the pre-Windows 10 way. This means that Windows 10 detects the settings as corrupted and throws them away, causing the handler to fall back to the system default.
The customer has a few choices here.
The obvious choice is to stop using Program X. Easier said than done. Program X is probably essential to the customer's workflow. That's why they want to set it as the default!
Another option is to work with the vendors who produce Program X and get them to stop mucking around in internal registry keys. (Good luck with that.)
Yet another option is to set the system default handler to Program X, so that when Program X accidentally-on-purpose corrupts the user preferences, Windows 10 falls back to the system default, which now happens to be Program X.
It does mean that the user won't be able to change their default handler to anything other than Program X. But given that Program X is corrupting the user preferences, this is the best you can hope for, assuming you are required to keep using Program X.