Mobs spawn at Y=256 where block light does not spread

Mobs can spawn at Y = 256, but block light does not propagate above Y = 255. I think both (a) mobs spawning above the build limit, and (b) block light not propagating to the top of blocks at the build limit, are bugs.

Steps to reproduce

1. Make a large glowstone platform at Y = 255.
2. Set time to midnight.

Expected result

No mobs spawn on the glowstone.

Actual result

Monsters spawn on the glowstone.