My Surface Book (i7, GPU 512GB) has gone back and forth between sleeping and not sleeping as various updates have been applied. Some updates fix the problem and then later another will break it again. However, everything seems to be working perfectly since the Anniversary update. I'm optimistic this might be the end of the issue.
I should add, though, that this doesn't prevent it from sometimes getting a little warm even when it's sleeping, nor from using up some of the battery while it's sleeping. But I haven't seen it lose more than 6% of battery while sleeping since the update. That's not a problem for me, especially if I'm getting background updates and backups and so on for that 6%.