I faced the same issue here. I think the problem is MS new default sound scheme for 5.1 includes side speakers as the last 2 channels for a 5.1 setup, rather then rear speakers which is the correct setup for 5.1. When you select 7.1, then the rear speakers are available. If they would simply reverse the side channels for the rear channels for 5.1 scheme, then this wouldn't be a problem for anyone. Having said that, the fact this has been an issue for this long, I highly doubt it will get fixed, and it's simply something you'll have to live with as most bugs with Windows 10 are.
This is actually not true, the standard setup for 5.1 is to have a pair of side surround speakers, 7.1 adds the rear surrounds.