All of a sudden, mobs started spawning in my well lit house.

After walking around a bit, I noticed that on stair blocks, light level drops to zero if you stand on the portion of the block that is cut out. This allows mobs to spawn in well lit spaces containing stairs.

In the image "Block Sky 0.png" you can see that Block and Sky are 0 in broad daylight.
In the image "Block Sky Normal.png" you can see that Block and Sky are back to normal if you move off the bugged area.

I've tested several step types and all are affected..