This problem has worsened day by day. Like many other Windows 10 features, the behaviour changes daily.
For years I’ve used Firefox or a derivative as my default browser. Recently that has been Cyberfox. Between December and March, that was fine. However, in March, Windows forget that Cyberfox was my default. It vanished from the browser list, and Cyberfox then kept bugging me to set it as the default browser. I couldn’t, as it stopped showing up in the list.
When I clicked on a link in my email program (Eudora), I had to select my choice of browser each time. Strangely, if I selected ‘Internet Explorer’ (not Edge), Windows 10 would assume that that was Cyberfox, and the links would open correctly. However, IE doesn’t appear in my browser list, probably for the basic reason that it isn’t installed.
The list of browsers has been shrinking by the day. Edge, still installed, vanished from it today. Basically, it seems the list of browsers I can choose shrinks day by day and my actual choice won’t stick.
The fourth image would look just like the first: no default is selected.
I have to say this is the most forgetful operating system ever. This isn’t the only place where Windows 10 has forgotten settings, or, for that matter, the choices I should have among those settings. I’m surprised your own Edge browser is no longer in that list.
So: what is the solution? Is there a registry tweak somewhere?