How to reproduce

1. Create a new world
2. Execute /fill ~-20 200 ~-20 ~20 200 ~20 dirt
3. Execute /setworldspawn ~ 200 ~
4. Execute /kill @s

Expected result

You should respawn in on the dirt platform you generated

Actual result

You spawn in underneath the dirt platform

I tested this in 1.12.2 and 18w16a and neither of these versions have this same issue so it has been broken recently

---

Very strongly related is MC-130138.
There the player spawns at the topmost block of an empty world, but inside a ship-wreck, which probably gets placed after calculation of player spawn point.