I noticed the same thing. It's probably a bug. Prior to BETA 2, if you have a headset of any sort connected, you're in a call, when you press the power button it just locks the screen.
With BETA 2, what happens is if you're in a different app or on the Home screen, it actually ENDS the call, which was annoying the heck out of me yesterday.
So, my pro tip (for now): If you're using a headset (wired or wireless), and you want to lock the screen/phone while on a call, make sure to go back into the Phone app first, and then hit the power button. Then it will keep the call going.