2015 12 inch MacBook, same problem.
In my case I have used my laptop for a long time so I think the battery is slightly degraded so I always have to keep it plugged into power to do time machine back up… Therefore it seems like I may not be able to do back ups until the next beta. I will see if I can attempt a time machine back up while I’m battery power but I did notice a couple issues where the laptop claimed it needed to charge when I tried to turn on the machine, and then upon hooking up the power cord it was at 61% battery… So my battery probably needs serviced on top of this being an affected 2015 model.
My trackpad freezes when I am trying to do a time machine back up, however I don’t think the battery would last long enough for the large back up I need to do. At first I thought it was because of an overheating issue because the bottom got really warm, about to hot to touch. But even having a cooling pad or even an air conditioning blowing against it, it still kept freezing. And when I came back, whether it was the display turned off or the screen was still on, the entire system was frozen including trackpad.
I will try a time machine back up with power nap disabled and without any power source connect into it. I will do it with 100% charged and see if the system freezes including trackpad. I will mention that the screen image remains the same even when the laptop lid is completely closed.