Steps to reproduce:
1. Download the attached project
2. Build to iOS
3. Make sure your device doesn't have rotation locked and rotate it to landscape (note: the app won't rotate as it is locked)
4. Press the button in the middle of the screen
5. Share to Notes
5.1. If there's no "Notes", then Press "More"

Result: upon sharing to another app, Unity app rotates to landscape mode
Reproduced on: 5.3.4p2, 5.3.8f1, 5.4.5p1, 5.5.3p3, 5.6.0p1, 2017.1.0b6
Devices used to reproduce: iPad Mini 2 (iOS 9.2.1), iPhone 6 Plus (iOS 8.0)
Devices that did not reproduce the issue: iPad Mini 2 (iOS 10.3.1)

Notes:
* Issue does not occur on iOS 10 (device always stays in portrait mode)
* On versions 5.3 & 5.4 the device gets stuck in landscape rotation, but in 5.5 and above, it is possible to get back to portrait mode by rotating the iPad and then it stays locked.