Regular mob spawning (not spawner or similar) doesn't happen on any height higher than the top block in that column that was naturally generated.
For example, in the Nether, nothing spawns at height y=129 or up, in the Overworld mobs don't spawn on a platform built next to a mountain, but they do spawn on the mountain.
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It also completely breaks mob spawning in Skyblock-esque maps, or at least the ones made with the void preset.
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Even making a new superflat filled to the brim with blocks and copying the region files over from the void world doesn't fix it.
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I also haven't had any phantom spawns.
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今後のマインクラフトに関わる重要なバグであり、早急に修正する必要があると思われる。
This bug is an important bug related to future Minecraft and it think that it is necessary to fix it as soon as possible.
qwerty23495
Not sure if I'm misunderstanding this report, but I can't reproduce it in 18w47b.
The red platform is at a level on or below the height of the originally generated terrain, while the blue platform is well above the ground, yet both spawn mobs.
FaRo1
Confirmed fixed in 18w47b. Was pretty hard to reproduce in 43c again, since light is totally messed up in that version, so I couldn't use exactly the same setup in both versions, but it should be close enough.
Fry