I was experimenting with the custom presets and generated a world where the terrain generates right up to the build limit. Most of y=254 is completely filled with scattered pockets of terrain at y=255.
On these scattered pieces of terrain at y=255 mobs can spawn in full daylight and don't start burning until they drop to y=254.
I didn't expect mobs to spawn in full daylight at this y level.
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Duplicate of MC-68565
Confirmed. I am guessing this is because half of the mob is not actually in the world, and mobs which aren't in the world presumably don't burn or don't have the same spawn mechanics.