This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
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 toAnswerHub just mention UE-25112 in the post.