HighResShot Alpha always 255

Expected behavior is that screenshot alpha should be 0 for pixels that did not have anything rendered into them.

1. Make a simple level with a cube, a directional light, and a player start node that's looking at the cube
2. In the level blueprint, on begin play, take a high res screenshot
3. In project settings, enable Engine -> Rendering -> Alpha Output
4. The output screenshot's alpha is uniformly 255, even though nothing was rendered in the pixels that weren't the cube