Reproduction Steps:
1. Import attached project.
2. Select "A" image in Project.
3. In Inspector, mark "Alpha is Transparency" as active.
4. Make sure Compression is set to "Normal Quality" or lower.
5. Apply changes.

Expected result: Alpha becomes transparent.
Actual result: Alpha becomes white.

Note: When set to a higher quality, alpha becomes distorted.

Reproducible with: 5.6.0f1, 2017.1.0a3
Not reproducible with: 5.5.2p3, 5.6.0b10
Regression introduced in: 5.6.0b11