Summary:
The player can respawn in the air if the spawnpoint is on a dangerous block.
If the spawnpoint is on a dangerous block, which is also the lowest closest point, the player will be teleported to the surface, as expected. However, if there are nearby blocks only at the level of the dangerous block or below, the player will appear in the air and may even take damage (and in some situations even death).

Steps to reproduce:

1. Create a flat world.
2. Replace one grass block with magma.
3. Place a spawnpoint on the magma block.
4. Kill yourself.

Observed results:
The player appeared in the air.

Expected results:
The player will appear on the surface.

Video:
0420(1).mp4