Not sure if it’s an iPhone 6/6 Plus issue - probably more of an iOS 8 issue.
I’ve noticed that when I double tap the home button to get to the App Switcher, you can no longer change the screen orientation. If you enter the App Switcher in portrait mode, you cannot go to landscape mode. If you enter the App Switcher in landscape mode, you cannot go to portrait mode. You have to exit the App Switcher before the display rotation occurs.
It’s not necessarily even a problem, more of an observation. FWIW, I did see some issues with the screen getting wonky when doing the in App Switcher screen rotation as late as Beta 5. Perhaps it s just easier to restrict the orientation than kill that bug.