I've had this problem since the iOS 9 beta and it still happening. It happens when you have are you using the landscape mode app and something else get swap too. I have to pulled up unlock the rotation and switch it back and then lock it again. It's something I've just learned to live with lol
Ok, let's get this clear because we are talking about the same bug, but from different angles and I understood why I was kind of wrong. It was a big bug at the beginning and now they just split it, partially fixed it, suggesting they are working on it.
You're definitely right: switching app from a notification, when it appears at the top of the screen or from the Notification Center pulled down, still ends in a landscape stuck thing (I didn't notice it before your feedback, so thanks).
Me, I was talking about the same initial situation but when switching app from the App Switcher. Since iOS 9.2b2 and from my usage, everything seems to be back like it used to be in iOS 8 and earlier, so now I can live with rotation lock alway on with no rotation issues using App Switcher from landscape mode only apps. Even the previews are back in portrait and don't show a outdated landscape preview when screen rotation is locked and the switcher opened from a landscape mode app.
But making tests to understand why we don't end to the same result just made me notice that if an app (let's say Mail) has previously been opened and closed in landscape (with rotation unlocked or also with the bug you've described), when you later lock the rotation, the App Switcher opens the concerned app in the orientation it has previously been closed (by close I mean going back to home screen with home button). So even from App Switcher, unlike what I thought, there is still a way to get stuck in landscape mode.
In short, when switching apps from a landscape situation with rotation locked:
- from notifications or Notification Center, apps still get forced and stuck in landscape mode [NOT FIXED]
- from App Switcher, apps are previewed, opened and stuck in the orientation they previously been quit and are not systematically previewed and forced to open in landscape anymore like it was the case in iOS 9 before 9.2b2 [HALF FIXED]