Board Thread:Bug Reports/@comment-27516710-20160821232722/@comment-27516710-20160822043424

I think it works both ways. The thing is that this spike is really bad and is compounded by having so many in one area (which is why Mojang cut back on the size of jungles, etc. due to the same problem). It's at it's worst the first time an area generates; once the lighting has updated and the shadows ticked away then the next time going there is fine.

Here are some more examples:

http://www.minecraftforum.net/forums/support/unmodified-minecraft-client/1853532-dark-shadow-bug