if you do /setworldspawn on a floating island in an amplified world, then do /kill, you spawn below the floating island instead of on it. If you do /gamerule spawnRadius 0, then that fixes the problem only if there are grass blocks in the floating island directly below the spawnpoint. If it is on top of a different block (bricks in my case) then you would still spawn below the island.

What I expected to happen was...:
i would spawn on the island

What actually happened was...:
i spawned below it instead if the block below the world spawn was something other than a grass block

Steps to Reproduce:
1. make an amplified world with cheats (i havent tested if the bug occurs in other types of worlds)
2. in creative mode, fly up above the ground and use /setblock ~ ~ ~ minecraft:bricks
3. stand on the bricks and do /setworldspawn
4. do /gamerule spawnRadius 0
5. do /kill and you will respawn below the bricks instead of on it
6. now put a grass block on top of the bricks and /setworldspawn on top of the grass block
7. do /kill and you will respawn on the grass block. Basically setworldspawn doesnt work properly if there is no grass block