Sun light passing through blocks

1. build a room far up in the sky (next to max height), corners not included, first made of slabs then replaced with blocks (mobs should spawn as there is was no lighting)

2. the room will be dark during the day, at first, and then at some point it somehow allows sun light to pass through the walls/floor/ceilings of the room (partially) (therefore mobs won't spawn and will burn if exposed to the part of the room with bugged lighting)

This was supposed to be my mob spawn grinder, but about 3/4 of it can't spawn mobs!