The reason Microsoft do this is to respect the user preferences[1]. When a program goes through the proper channels, Windows pops up the dialog which says "you have installed a new program which can handle {Foo} filetype, would you like to open it in the new program, or the one you were using?"

When a program skips this user-respecting dialog, and edits the registry directly to take control of a file extension without user consent, Windows detects it and resets, to give control back to the user instead of letting random software take over. It's part of Raymond Chen's "What if two programs did this?[2]" and "I bet someone got a nice payrise for hijacking user file extensions" comments on his OldNewThing blog.

[1] https://devblogs.microsoft.com/oldnewthing/20170517-00/?p=96...

[2] https://devblogs.microsoft.com/oldnewthing/20050607-00/?p=35...

[3] See also: https://devblogs.microsoft.com/oldnewthing/20190724-00/?p=10...

I really don't understand your reasoning.

Let me clarify the scenario:

1. the user modified the default application for PDF to use FoxIt.

2. Win10 reverts the assignment back to Edge after an update and/or reboot.

There is no other program involved. There is a user setting the preference and Win10 ignoring it, consistently for a period of several years, ever since I installed it.