You can see in the screenshot that although the edit sign text has been brightened, the actual sign text is unchanged in it's brightness.

I believe the screenshot you have given doesn't relate to the issue you are talking about.
Maybe you uploaded the wrong screenshot?

Fixed it, I think. I tried to recreate it, and it didn't occur on a brand new world, just the old one. I did notice some modded signs don't work correctly, some not at all, some glow but don't change the color of the text.