Steps to reproduce:

1) Download attached project 'TestTextures.zip' and open in Unity
2) Check if platform is Android in Build Settings
3) Make sure Texture compression is ETC(default)
4) Reimport 2 textures which are in the Assets folder to apply compression
5) Click on 'grass_normal.png' texture and check preview window
Note: Compressed texture is ETC2(check attached image 'ETC2.png'), even compression is set to ETC(default)

Expected result: Compressed texture should be ETC(default)(check attached image 'ETCdefault.png')
Actual result: Compressed texture is ETC2, even compression is set to ETC(default) in Build Settings

Regression since: 5.5.0p4

Reproduced with:
5.5.0p4, 5.5.2p2, 5.6.0b11, 2017.0a3

Not reproduced with:
5.4.4p4, 5.5.0p3

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Fixed in: 5.5.4p2