Reproduction steps:
1. Install RenderDoc on your machine
2. Open the attached project "ReproProject"
3. Ensure that DirectX Graphics API is active (Edit > Project Settings > Player > Graphics APIs for Windows)
4. Load RenderDoc (right-click on Game window title > Load RenderDoc), Capture the frame
5. In the RenderDoc Window, observe Pipeline State (select OM from the pipeline)
6. Look for UniversalRenderPipeline.RenderSingleCameraInternal: Camera > ScriptableRenderer.Execute MyUniversalRendererData > Render GBuffer > RenderLoop.Draw > DrawIndexed(36)
7. Under the Target Blends section of the Pipeline State, observe that the write masks are all G values
8. Switch the project to use Vulkan Graphics API (Edit > Project Settings > Player > Graphics APIs for Windows, make Vulkan appear above Direct3D11) and accept the request to restart the Editor
9. Repeat step 4
10. In the RenderDoc Window, Look for UniversalRenderPipeline.RenderSingleCameraInternal: Camera > ScriptableRenderer.Execute MyUniversalRendererData > Render GBuffer > RenderLoop.Draw > vkCmdExecuteCommands
11. Under the Target Blends section of the Pipeline State, observe that the write masks are all RGBA values except for index/slot 0

Expected result: The values for the write masks should be the same for both graphics (DirectX and Vulkan)
Actual result: The values for the write masks are not the same

Reproducible with: 2021.3.37f1, 2022.3.23f1, 2023.2.17f1, 6000.0.0b14

Reproduced on: Windows 11 23H2 (22631.3007), Windows 11 (10.0.22631) 64bit (user’s)
Not reproduced on: No other environment tested