Steps to reproduce:
1. Open User attached project "EditorGUI.zip"
2. Inspect the Texture in the project window and set its type to "Normal Map"
3. Texture preview is reddish in color

Expected: Texture should be transparent and no red color appears randomly
Actual: The preview shows the texture shaded red and the preview in the project window sometimes breaks as well

Reproduced on Windows 10 D3D11 and D3D9, MacOS 10.12.2 Metal and OpenGL

Reproduced on Unity versions: 2017.1.0b4, 2017.1.0b8, 2017.1.0p2, 2017.2.0b5, 2017.3.0a2
Regression introduced in 2017.1.0b2
Doesn't reproduce on 5.6.3f1, 2017.1.0b1