To reproduce:
1. Open attached Unity project "AlphaHitTestProject.zip"
2. Open scene "testScene"
3. Go into play mode
4. Click on different places of "foregroundImage"
5. Observe the Console Window

Expected result: According to documentation, Image.alphaHitTestMinimumThreshold's value set to 1 would cause only fully opaque pixels to register raycast events on the Image. In other words, clicking on transparent (red colored) parts of the foregroundImage should result in BACKGROUND HIT console output and clicking on the opaque part (blue colored) should result in FOREGROUND HIT console output
Actual result: Clicking on most of the opaque space results in BACKGROUND HIT console output and clicking on some transparent space results in FOREGROUND HIT console output

Reproduced on: 5.5.3p3, 5.6.1p1, 2017.1.0b6