I have the same problem as everyone else above and I am fairly new to working with my MacBook Air and magic mouse. I was in contact with Apple three different times and tried all sorts of solutions to get the mouse to scroll but pretty much the fix that worked best was turning the mouse off and on. It's hard because the button is so tiny on the bottom and it's just a pain. The last representative I spoke with sent the following email a few days after I spoke with him:
I received an update from our engineers on our case regarding the Magic Mouse losing connection to Mac after it goes to sleep. The engineers have reports of the issue and it is currently under review, the resolution may come in the form of a update for macOS but as of right now there is no official fix. For the time being continue to use workarounds until a fix is pushed out via software update.
Sounds like they are working on it and I am glad I am not alone. I was actually thinking of returning my mouse and getting another one, thinking the problem was with that. Hope to find an answer soon.