Run attached test. It shows a frame with 4 components. Each component draws two rectangles in different modes: with opaque/translucent buffered image and with AlphaComposite alpha value of 0.99999f/1.0f
When the test is run with the latest 7.0 build, with D3D pipeline enabled, opaque/0.99999f component is rendered incorrectly (top-right component): buffered image is not composited with white area at all. Attached screenshot Test2.png shows the final picture, and Test2-correct.png is a right frame (when D3D pipeline is turned off).