Try opening a PDF file, you can choose an app to open it with (for example firefox) but you can’t change it after you set it as open with “always“.
This is annoying. I want to be able to switch to a different PDF reader as default.
Maybe it is a bug but for clarity are you unable to undo as
Settings > Apps > See all apps > the_problem_app > Set as default > Clear Defaults.
This support topic suggests the fix could be in Firefox settings Choose your preferred PDF viewer in Firefox | Firefox Help.