This world was actually built on the 1.1 version i just updated my MCPE to 1.8.0 and i found this bug that makes half slabs not allow light through. I tried replacing them but after exiting it just goes back to now allowing light through. And by the way i have my brightness on full. Even if i just go far away from the replaced half slabs it turns back to registering as solid block. I'm using a Xiaomi Redmi Note 3. Hopefully this issue will be gone in the incoming updates of MCPE. Thank you.