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.).