I really do wonder what causes the unevenness of this. For me, I installed Foxit reader and it's always stayed my default. I also switch .txt files to open in Notepad++ and it has always stayed the default too. I've changes a lot of file extensions over the years and they always stuck through all the Windows updates, etc... I've never used any version of Edge as my main browser nor has it ever become the default for anything automatically. I may have been prompted to try it during an upgrade welcome screen but I've always said no and it's never bothered me again.

A/B testing has been de-rigueur on the web for a very long time now. What makes you think Microsoft aren't playing the same tricks?

In my experience, this is Edge updates doing this.