Put the summary of the bug you're having here
What I expected to happen was...:
I would spawn on the floating block when first-joining
What actually happened was...:
I spawned next to the block instead of on top of it and died.
Steps to Reproduce:
1. Create an empty, bottomless world
2. Fill in one block of your choice
3. Set the world spawnpoint on that block
4. Set the world to public
5. Join the world with an alternate account
6. You will spawn off of the block and fall to your death
I'm pretty sure that's not a bug. According to the Wiki, unless you're in adventure mode, players will spawn within a 20x20 (or, in multiplayer, 10x10) area around the world spawn point. If in adventure mode, they'll spawn exactly on it. That's intended behavior.