Summary
The player can respawn inside leaf blocks, provided that the player's spawnpoint/world spawn has never been changed.

Steps to Reproduce (new):

1. Create a superflat world with immediate respawn turned on (to facilitate faster respawning).
2. /fill ~-5 ~ ~-5 ~5 ~2 ~5 leaves
3. Repeatedly commit suicide (/kill).

Observed results:
The player always spawns in the leaves, standing on the grass level.

Expected results:
The player should spawn on top of the leaves, or outside the leaves.

Original reproduction steps:

1. Create a regular world with the seed

   590477318

   with cheats enabled and Respawn immediately/doimmediaterespawn turned on (to facilitate faster respawning). The player should initially spawn at [500, 85, 4] (on top of a tree).

1. Repeatedly commit suicide (/kill or whatever).
2. Eventually the player should spawn inside leaves, partially or fully. In these instances (more than one leaf spawning spot!) note that the block the player spawns on is always a tree log, when you clear the leaves.