Reproduction Steps:
1. Import attached project.
2. Go to the Assets folder in your project files.
3. Observe how the "a-upper" is an RGBA image.
4. In Unity, select and reimport "a-upper" in Assets.

Expected result: Image is imported as an RGBA texture.
Actual result: Image is imported as an RGB texture.

Reproducible with: 5.5.2p1 - 5.5.2p4
Not reproducible with: 5.5.2f1
Regression introduced in: 5.5.2p1

Note #1: If Compression is set to "High Quality", the texture becomes RGBA. However, the alpha still seems to have issues. (Image attached)

Note #2: This is a known 5.6 issue that has been resolved (see case 890681). However, it seems have been back ported in 5.5.

Note #3: 16bit textures(per channel) alphas are not imported at all.