Slab XRay

Now I don't know much, and i may be wrong, but looking at those pictures it appears you are in the slab, not below the slab, and I believe slabs are transparent blocks (like glass and glowstone) so going in a slab would make you see through it, especially if  you can only see through the slab.

From your screenshot you are seeing through multiple slabs, can you upload a screenshot of you not seeing through other blocks?

Sorry I'm French and I don't understand what you want Urielsalis

Yes Gavin the slab is a transparent block but it's solid so we should not see through it

Place blocks next to the slab and show us screenshots to check if you see through those blocks or not, try with stone(a solid block) and glowstone(a transparent block)

I can't take the screenshot now but I know I can't see through solid block I tested it 

Post them when you can take the screenshots