FlatGenerator from example not working properly since release 1.13

[anddero]

Running the example in the readme and opening the world with Minecraft Release 1.13 or later won't work properly. Version 1.12.2 worked as expected, but 1.13 and later versions seem to think that the flat world layers of bedrock and melon are layers of air instead. Was there a change in the material codes of melon and bedrock? Is it an issue with the flat generator? Something else?