There is full light everywhere, even without light sources. It has been happening in all of my worlds and servers. I have been using a hacked client on anarchy servers which includes a light hack, but the bug also happens in my vanilla survival world
MC-182051
Full light in closed off areas with no light source
Resolved
Invalid
0
If I'm not wrong, when you use "fullbright" in that client, it actually just changes the "gamma" area in the options.txt folder to a high value, which carries over to all versions that use the folder the specific options.txt is in.