Steps to reproduce:

1) Open Unity and create a new project
2) Make sure orientation is set to landscape left/right in the Player Settings
3) Build and run project on a device
Observe black borders on the screen

Expected result: no black borders (check attached image "expected.png")
Actual result: black borders appears if orientation is set to landscape left/right (check attached image "actual.png")

Reproduced with:
5.6.5f1, 2017.1.3f1, 2017.2.1p1, 2017.3.0f3, 2018.1.0b1

Devices under test:
Reproduced with:
0138 Samsung Galaxy S8, OS: 7.0, CPU: Samsung Exynos 9 Octa, GPU: ARM Mali-G71
0154 Samsung Galaxy Note 8, OS: 7.1.1, CPU: Samsung Exynos 9 Octa, GPU: ARM Mali-G71

Not reproduced with:
0150 Google Pixel XL, OS: 8.0, CPU: Snapdragon 821, GPU: Adreno 530
0162 Razer Phone, OS: 7.1.1, CPU: Qualqomm Snapdragon 835, GPU: Adreno 540

Note: Works well if orientation is set to autorotation