To reproduce:

1. Download and open attached "Android Mask Bug.zip" project.
2. Build and run on a device.
3. Notice the initial green rectangle with blue stripe in the middle.
4. Press VR button to enable Virtual Reality mode.
5. Notice how blue stripe changed it's shape.

Actual result: Mask component is disabled. Blue stripe in the middle streches out of green rectangle's limits.
Expected result: Mask component stays enabled. Blue stripe stays in it's original position and does not change it's shape.

Reproduced with: 2017.2.0b1, 2017.1.0f2, 5.6.2p1.

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 G903F*, OS:6.0.1, CPU:armeabi-v7a, GPU:Mali-T720,
Build:samsung/s5neoltexx/s5neolte:6.0.1/MMB29K/G903FXXU1BQC1:user/release-keys

Google Pixel*, OS:7.1.2, CPU:arm64-v8a, GPU:Adreno (TM) 530,
Build:google/sailfish/sailfish:7.1.2/N2G47E/3766409:user/release-keys

iPhone 6s iOS 11.0

Notes: Tested with Daydream, Cardboard and Oculus VR SDK's. Did not reproduce on Oculus VR SDK. Actual result can also be seen in the editor if Mask component is disabled on Canvas/Image gameObject.

Works with 24-bit buffer selected.
Verified with 5.6.3p1, 2017.1.0p4.