The caves world type uses netherrack as default block and lava as default fluid, like the Nether.

The floating islands world type only generates one island in the center of the world and all other islands generate 1000 blocks out (exactly like the End). The default block is end stone and default fluid is air, like the End.

All of this happens because when selecting "Caves", the world type is set to "minecraft:nether" instead of "minecraft:caves", and with "Floating islands", it is set to "minecraft:end" instead of "minecraft:floating_islands".

Using custom worlds I could create the expected result (what buffet worlds would generate).