Game treating certain blocks as glass; light goes right through them

In one of my worlds (I've had it since about 1.12) I have a mob spawner. The entire thing is made of cobblestone and since the snapshots came out it has stopped working. I looked up there and despite having a full layer of blocks covering the top, the entire thing is filled with light without any openings or light blocks in sight. When manually spawning mobs in they burn even with a roof over them. I tried adding another layer and that didn't change anything.