UI transparency breaks when dragging a game window around the screen

When a game window launched from UE is moved around the screen, any transparent elements on the screen will turn fully opaque. The loss of transparency also causes some text to become blurry. This does not happen in PIE, only when the game is launched.

Found in 4.18.1 3658906. Reproduced in 4.19 3764500 and 4.17 3658906

1. Create a new project with a partially transparent object in your UI
2. Launch the game (Do not use PIE)
3. Drag the window around the screen aggressively without releasing the mouse button. The transparent object will become fully opaque. Once you release the mouse button, everything returns to normal.

Expected result: Transparency still displays correctly even when the window is being dragged around.