Here is why:
When you select show windows Side by Side it shows you all of the windows that are not minimized so you may select which two you want to view side by side:
Pick one and drag it to the right side of the screen
Click somewhere on the screen.
Pick another and drag it to the left side of the screen.
Now you have 2 windows displayed side by side.
The function has been changed and steps added, but it would seem this is to include further functionality. (You can pick and choose your windows and have more than two open and even full screen)
Life is change, we can either roll with it... or become jaded. This is not worth getting overly upset about. It is unfortunate that many of us did not know this or where to find the info.
(Try this, open four windows. Make two full screen
and two viewable but not full screen.
Select view windows side by side. (all windows seem to become tiled.)
Select one window and drag it to the right. (Leave at least one full size window alone)
Click and empty space on the screen and then
Select another window and drag it to the left. (Leave at least one full size window alone)
Notice the windows you have chosen are side by side
Now you have windows side by side.
HOLD ON, there is more.
Now on the task bar, select one of the fully expanded windows.
You see the window is fully expanded again.
POINT: You can have 10 windows open, select two to view side by side, then the rest should keep their original view settings BEFORE you decided to view two side by side.
I hope that someone will hit "answer" to put this to rest for over 11K people.