Player spawns on a bedrock layer when the world is first created as a SuperFlat world with honey blocks on the surface.
Non-full blocks:
Honey block
Dirt path
Farmland
Snow
Soul sand
Mud
Reproduce
minecraft:bedrock,10*minecraft:honey_block;minecraft:plains
Related issues
discovered while testing
relates to
Attachments
Comments

[media][media]
I can confirm this behavior.
[media]Fixed with MC-272650.