some chunks turn darker when entering the nether for a while

i entered the nether for a moment when i was back i found this wierd chunk that turn darker
while there is nothing above it
there is a tree i made it but there is no block above that chunk
i tried to break blocks and add light source but that chunk still dark
i put a view from up so you can make sure there is no block above it
that chunk was't dark at all before i enter the nether it was normale
and if anyone know how i fix that dark area tell me because i can't fix it till now

see the screenshots for more