Reproduction steps:
1. Open the attached project ("case_1227481-VulkanRepro.zip")
2. Open the repro scene ("Scene")
3. Inspect the gradient texture generated by compute shader
4. In Hierarchy, select Main Camera
5. In Inspector, find "Test Renderer (Script)" component
6. Change "Copy Mode" to "Alpha Only"

Expected result: Alpha channel should remain unchanged, as it was copied
Actual result: Alpha channel data changes (game view becomes black)

Reproducible with: 2018.4.20f1, 2019.3.7f1, 2020.1.0b4, 2020.2.0a5
Not reproducible with: 2017.4.39f1

Notes:
1. The issue reproduces on Vulkan graphics API
2. The issue does not reproduce on DX11, DX12 graphics APIs
3. If _CopySource type in TestCompute.compute is changed to Texture2D<float4>, alpha works as expected.
4. The issue happens only for B and A channels - if R or G are preserved instead, they work as expected.
5. Alpha is probably not really zeroed - in more complex cases channels are modified, but still based on default values (e.g. color shift, value range change).