I agree, I don't think these are necessarily light diffusion layer issues. If you look at both the before and after photos, there are two more pronounced white dots on the screen farther down. The OP doesn't complain about them, but they clearly show up as worse spots. They are likely just spots on the glass from some sort of liquid, perhaps even spit, that dried. No big deal, but as you can see photographing screens can amplify non issues into big issues. If the original bright spot that gave the OP issues has grown faint, then it clearly wasn't a bad part in the eInk layer or the diffusion layer, as they won't correct themselves. It is probably just schmutz on the screen. Clean the screen thoroughly and all will likely be better.

Note: The eInk screen is NOT backlit, it is front lit. However, any transparent or translucent spots on the outer surface of the glass screen will be backlit, thus making them glow. Eyeglasses or camera lens cleaner will correct these.