The Alpha Channel of the Render Target Texture is rendered as a value of 1 (solid white) when the SceneCapture2D actor is set to LDR with PP and Anti-Aliasing is set to true in the General Show Flags. With Anti-Aliasing set to false the Alpha Channel renders correctly.
Reproduced in Dev-Rendering CL 2817080 and 4.11 Preview 1
*Quick Reproduction in Attached 4.10.1 Project*
RESULTS: In the AA=True (Default) Scene Capture, the Alpha Channel renders as a value of 1 (Solid White)
EXPECTED: Both Textures retain the Alpha Values shown in the AA=False RenderTarget Texture
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-25112 in the post.