Spawners count the interior of solid blocks as adequate darkness to spawn mobs

When I placed a custom zombie spawner in my world, the spawner seemed to count the interior of solid blocks as both a place that was dark enough and adequate room for the zombies to spawn, causing them to be spawned inside the blocks during the day.

I expected the custom zombie spawner I had placed into the world to spawn zombies on the ground, on top of the blocks.

Instead, the zombies were able to spawn inside the solid blocks that surrounded the spawner, despite the fact that it was daytime.

Steps to Reproduce:
1. Place a spawner near solid blocks. (I used a custom zombie spawner.)
2. Set the time to 6000
3. When the spawner tries to spawn a zombie nearby, the sunlight makes it too bright for the zombie to spawn on the surface, so it spawns it inside solid blocks instead, due to the fact that no light can reach the inside of a block.