Minecraft wiki entry for Pillager_Outpost, associated text: In Java Edition, if an outpost spawns in the water or in the air (e.g., spawning near a savanna plateau), then a platform of grass (or sand), dirt, and stone spawns underneath it in a circular shape. This also applies to the smaller structures such as tents, targets, log piles, and cages. On occasion, the smaller structures still spawn in the air despite having a platform below them, depending on how high up they are.