time machine backup files still open?

Not exactly. But while Time machine is on automatic, it always is looking for changes to backup.

If you turn off time machine through System Preferences or add its menubar icon from System Preferences to be able to turn it off, it will constantly be on, looking for changes in the area desired. If you step back in the time machine you will see completed backups in the time machine folders with non-grayed out icons.

Anything in the .TimeMachine folder is in the process of being backed up. These are temporary files that aren't for your access.

Well the "should" part about TIme Machine unfortunately we have little control over, as the absence of such controls on the Time Machine settings page would indicate it goes by its own logic. Which is fairly automatic. And part of the issue, since you haven't restarted, it doesn't know what you want to do. When you do restart, that's when it closes everything out and starts up again.

You are welcome to submit feedback as an Apple developer or via http://www.apple.com/feedback/