New
#21
I think this is what you're experiencing - just came across this news article and posted it - with a possible reg fix (note caveat).
However you may prefer to uninstall the update and hide it so it doesn't recur (see Tutorial section on how).
MS admits Windows 10 Forced Update Resets Default Apps - Windows 10 Forums
....don't try to change associations from within programs- they won't 'stick'. (e.g. cab)