Reproduction steps:
1. Open the attached 'texture test 2021' project
2. Open the 'SampleScene'
3. Observe the 'Quad' GameObject

Expected result: The 'Quad' GameObject is completely black with a white stripe at the bottom
Actual result: The top half of the 'Quad' GameObject is white

Reproducible with: 2021.1.0a1, 2021.3.1f1, 2022.1.0b16, 2022.2.0a11
Not reproducible with: 2019.4.38f1, 2020.2.0b5, 2020.3.34f1

{}Important{}: The "actual result" mentioned above is expected and by design due to a previous optimisation. What the fix focuses on instead, is clarify that the user should not rely on the colour data of texels where Alpha == 0 when Alpha Is Transparency is enabled. (this applies to 2020.3 as well, before the optimisation change was made)
As such, when verifying, you should check that the tooltip for "Alpha is Transparency" & API docs have been updated as seen in the PR, instead of checking the repro project scenario.