Hi,
That is normal MS policy to reset defaults to own apps on updates not just with MMW. Just change it back and do not worry.

I get a warning in the Action center
app default was reset
An app caused a problem with a ..

reset to default application Groove Music

There is no log of Action Center Messages. I have not seen anything in Event Viewer as yet that gives me any more info.

I am not sure if this is a bug. It reset the default application to play mp3 and flac files