Just reproduced this. Yep! Wow, kinda rare to see such a substantial new bug this far into the beta cycle.
EDIT: Interesting. Lock/Power button doesn't end the call if you have it on speaker or headset. Only when using the earpiece.
EDIT AGAIN: Maybe this isn't a bug. When you're on a phone call with the earpiece, what reason would you have to press lock/power? You'd only want to press that to turn off the screen if you're on headset or speaker... So perhaps Apple added this as a way to quickly hang up a call if you're using the earpiece. Not the most intuitive thing in the world, but.. you never know!