How to reproduce:
1. Open the attached project
2. Build and run Main scene on device
3. Press "Portrait Upside Down" button
Actual result: Scene's orientation is Portrait
Expected result: Scene's orientation is Portrait Upside Down

4. Press Autorotation button and enable all orientations on the right
5. Orientate device upside down
Actual result: Scene's orientation is Portrait
Expected result: Scene's orientation is Portrait Upside Down

Reproducible: 2018.3.0a4, 2018.1.7f1, 2017.4.7f1, 2017.1.3p3

Tested and reproduced:
VLNQA00099, Google Pixel XL, Android 9, CPU: Snapdragon 821 MSM8996 Pro, GPU: Adreno (TM) 530
VLNQA00110, Samsung Galaxy Note2 (GT-N7100), Android 4.4.2, CPU: Exynos 4 Quad 4412, GPU: Mali-400 MP

Tested and didn't reproduce:
VLNQA00104, Lge Nexus 5 (Nexus 5), Android 6.0.1, CPU: Snapdragon 800 MSM8974, GPU: Adreno (TM) 330
VLNQA00023, Xiaomi Mi 4i (Mi 4i), Android 5.0.2, CPU: Snapdragon 410 MSM8916, GPU: Adreno (TM) 405
VLNQA00025, Xiaomi Mi 5s (MI 5s), Android 6.0.1, CPU: Snapdragon 820 MSM8996, GPU: Adreno (TM) 530
VLNQA00122, Samsung Galaxy S9, Android 8.0.0, CPU: Samsung Exynos 9, GPU: Mali-G72
VLNQA00123, Google Pixel 2 XL, Android 8.1.0, CPU: Snapdragon 835, GPU: Adreno (TM) 540

BY DESIGN: This is by design/OS issue when it doesn't allow Portrait Upside Down to be locked.