/spreadplayers occasionally suffocates players

The spreadplayers command behaves much better than it used to, but it still sometimes suffocates players. Specifically, if you run

/spreadplayers ~10000 ~10000 50000 50001 true @s

about 100 times, about 5 of those times you'll find yourself suffocating in a block.

Every time I was suffocating, the behavior was consistent: the block above my head was a leaf block. I speculate that when spreadplayers sees that the topmost block at the x,z it wants to put you at is a 'leaf' block, it moves you down so that the players head is one block below the bottom-most contiguous leaf. However, leaves can be above wood or the ground, and so sometimes this means the player spawn in solid wood or dirt, suffocating.

(This is a relatively minor bug; the behavior now seems much improved compared to MC-52254 )