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
Make a large glowstone platform at Y = 255.
Set time to midnight.
Expected result
No mobs spawn on the glowstone.
Actual result
Monsters spawn on the glowstone.
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In 1.18.0 mobs can spawn on top of blocks at the new build limit (Y = 320), however, spawns there correctly take into account block light from nearby blocks and sky light. Resolving as Fixed.
The light level still does not show correctly on top of blocks at the build height, but that is now only a client-side bug. We will track that at MCPE-149213.
This issue was reported two years ago for version 1.8 at MCPE-39665, but unfortunately that report was overlooked and never confirmed.