To reproduce:
1) Open attached project and scene
2) Open RenderDoc and inject it into process
3) Get a capture (you can also check screenshots in the project folder
4) Navigate to Pipeline tab -> Output Merger and check the following: Camera.Render > Drawing > Render.OpaqueGeometry > RenderDeferred.GBuffer > last 3 calls.

• cube 1 uses One Zero on all RTs, which is the default
• cube 2 uses One OneMinusSrcAlpha on all RTs, as expected
• cube 3 uses One OneMinusSrcAlpha on RT0 as expected, and One SrcAlpha on RT3

but RT1 and RT2 use One Zero: that is NOT expected!

Reproduced in 2017.3.0f3, 2018.1.0b6