Just because you didn't experience it in 10.2.1 or B1, doesn't mean its not a thing. I first started experiencing the issue in iOS 4. I've experienced it on Windows, OS X and macOS. Its not hard for a processor to get stuck in the background, even if the apps have been closed out. The hard part is finding that processor. If you are using a Mac, download Xcode and run the instruments program. It will tell you what processors are running and you might be able to find the rogue on. I once had the App Store running at 75% CPU and was stuck. That was the cause of my problem.
You and applyr both think its a bug, but I'm saying its normal for this to occur. Its not ideal, but its not a bug or due to incompetence.