As the problem only occurs when rebooting of any kind, but not on a cold start, this may be because the X server starts before any information has been fed to it, and it is showing the contents of the framebuffer in the graphics card's RAM until that time.
As the problem only occurs when rebooting of any kind, but not on a cold start, this may be because the X server starts before any information has been fed to it, and it is showing the contents of the framebuffer in the graphics card's RAM until that time.