To reproduce:
1. Open attached project;
2. Open "VRScene" scene;
3. Build and Install it on an Android device;
4. Launch the XR_Rotation app on the device;
5. Let the app close on its own;
6. Repeat 4-5 steps for 5 times (or more, if no repro).

Actual result: InputTracking.GetLocalRotation(XRNode.Head) first returned value is: 90.00, 270.00, 0.00.
Expected result: InputTracking.GetLocalRotation(XRNode.Head) returns something like this:
Rotation: 88.78, 13.87, 103.87
Rotation: 88.78, 14.04, 104.04
Rotation: 88.78, 13.50, 103.50
Rotation: 88.78, 13.48, 103.48

Reproduced on versions: 2017.2.0a1, 2017.2.2p1, 2017.3.1p3, 2018.1.0b10, 2018.2.0a3.
Not reproduced on versions: 2017.1.3p2.

Devices under testing:
Reproduced with:
Samsung SM N950F*, OS:7.1.1, CPU:arm64-v8a, GPU:Mali-G71, Build:samsung/greatltexx/greatlte:7.1.1/NMF26X/N950FXXU3BRA8:user/release-keys
Samsung SM N950U1*, OS:7.1.1, CPU:arm64-v8a, GPU:Adreno (TM) 540, Build:samsung/greatqlteue/greatqlteue:7.1.1/NMF26X/N950U1UES2BQL2:user/release-keys
Google Pixel 2 XL*, OS:8.1.0, CPU:arm64-v8a, GPU:Adreno (TM) 540, Build:google/taimen/taimen:8.1.0/OPM1.171019.018/4518263:user/release-keys

This is a system level issue with Google VR itself. According to Google, this is as intended.