Steps to reproduce:

1) Download attached project 'iOS GVR Bugs.zip' and open in Unity
2) Build and run 'Main Scene' on a device
3) Hold device in horizontal position
4) Rotate it 45 degrees left/right
5) Click on 'Enable VR' button
Application will enter to VR mode(VRSettings.enabled = true;)
6) Return device to straight horizontal position
Camera will stay at ~ 45 degrees and will not return straight view

Expected result: Camera's view should be straight (check attached image 'expected.png')
Actual result: Camera stays rotated after VR mode is enabled (check attached image 'actual.png')

Reproduced with:
5.6.2f1, 2017.1.0b10, 2017.2.0a4

Devices under test:
Verizon SM G930V*, OS:6.0.1, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:Verizon/heroqltevzw/heroqltevzw:6.0.1/MMB29M/G930VVRU2APG3:user/release-keys

Samsung SM G928F*, OS:7.0, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zenltexx/zenlte:6.0.1/MMB29K/G928FXXS3BPIA:user/release-keys

Not reproduced with:
iPhone 6s iOS 10.3.1 and iPhone 6 iOS 10.2.1.

Reproduced with:
iPhone 5SE, iOS 10.0.1

RESOLUTION: By design. User modifies the rotation of the camera in the VR simulation and does not restore it before entering VR mode. After fixing this issue in user code the bug is no longer reproducible.