Reproduction steps:
1) Download and open reproproj.zip project
2) Build and run main.unity scene on an Android device
3) Touch the screen
-- Notice that video is scaled without preserving original aspect ratio

Expected result:
As FullScreenMovieScalingMode.AspectFit documentation claims: "The aspect ratio of the movie is preserved" so it is expected for aspect ratio to stay the same.

Reproduced with:
5.5.0f3, 5.5.5p1, 5.6.4p2, 2017.1.2p2, 2017.2.0p1, 2017.3.0b8, 2018.1.0a3

Devices under testing:
Reproduced with:
Google Nexus 7, Adreno 320 (1200x1920)
Google Nexus 7, NVidia Tegra 3 (800x1280)
Samsung Galaxy S8, Mali-g71 (1440x2960)
Samsung Galaxy S8, Adreno 540 (1440x2960)
Huawei P9, Mali-T880 (1080x1920)
Samsung S7, Mali-T880 (1440x2560)
LG G4, Adreno 418 (1440x2560)
Google Nexus 6, Adreno 420 (1440x2560)

Unable to reproduce with:
Xiaomi Mi PAD, Tegra K1 (1536x2048)
Sony Xperia Z4 Tablet, Adreno 430 (2560x1600)
Samsung Galaxy Tab 3, PowerVR SGX 544MP2 (1280x800)
Google Pixel XL, Adreno 530 (1440x2560)
Google Pixel, Adreno 530 (1080x1920)