When I used the buffet mode in 1.13.2, I created a swamp biome and used Floating Islands. I opened the world, and underneath the island there were weird structures made of bone block and coal. There were also incorrectly spawned Mineshafts scattered around the map.
MC-144411
Weird Structures in Swamp Sky Islands
Resolved
Works As Intended
0
Those "weird structures" are fossils, fairly rare structures that spawn underground in desert and swamp biomes.