Basically, trying to add more than one flat world to a world generation preset json causes the game to crash with

java.lang.IllegalArgumentException: value already present: chf@d5af7f7java.lang.IllegalArgumentException: value already present: chf@d5af7f7 at com.google.common.collect.HashBiMap.put(HashBiMap.java:287)

Using the same settings, but setting the overworld to be regular generation results in the world generating just fine.

Edit: Test 404 is the file with both the overworld and minecraft:test as flat worlds. Test 405 is just minecraft:test as a flat world.