I noticed this happening pretty early on in the beta cycle. I'm not sure if it was in beta 1, but I think it was definitely happening by beta 2 or 3. It only happens on the iPhone (not the iPad) and it also only seems to happen in apple apps (mail, messages, safari, etc). When you switch to landscape mode the whole status bar disappears.
When I first saw it, I thought it was a beta bug, but over the months I was using the beta I decided that I liked the feature. When the phone is in landscape mode, especially when the keyboard is up, there is very little screen space, so this gives you at least a bit more available space.
As far as I know, though, there is no way to turn it off. If you need to see the time you can either swap it into portrait mode or pull down the notification bar (or, in early 2015, look at your apple watch)
