Opaque blocks not acting like opaque blocks

Certain opaque blocks do not act as opaque blocks. It seems they are either transparent or emit their own light. Difficult to replicate, stops occurring after you destroy the block/replace it. Screenshot shows that the light level is 0, however mobs still don't spawn. At one point, I even completely encased myself in cobblestone, and there was still light. Issue reoccurs (I rebuilt everything, issue resolved temporarily. After rejoining the server, I had the same issue again.).