See PhoenixSC's video on this

To Reproduce:

1. Launch any MC version >= 1.13.2 (includes 1.14).
2. Click "Create New World"
3. World Type: Superflat
4. Click "Customize" then "Presets"
5. Select the void preset.
6. Click "Use Preset" then "Done" then "Create New World"
7. Inspect the platform's corners.
8. You will see that one of the corners (the south-east corner) is missing a block.
9. Repeat steps 1-8, but launch any MC version < 1.13 (1.12.2 and below).
10. You will see that the whole platform is normal (the south-east corner is properly filled in with stone and not air).

Possible Cause: Ethier (1) the logic for generating the void platform fails to place the very last (south-east) block, or (2) the structure file used to generate the platform is missing the block at the south-east corner.

May be related to MC-125872 (but that one says that the ENTIRE stone platform does not generate, which is different than just the block at south-east corner not generating)