Blocks in some chunks keep sky light of 15 even when there is no sky acces...

Built a creeper farm on a server. The server is full vanilla but when i unload/load the chunks some of the blocks give light lvl 15 from the sky. I need to break and replace half of the chunk on every layer and the roof of the farm to be able to make it work and some blocks still get light lvl 10 when all the blocks around have less light...