I am trying to work out the max height of a chunk, so rather than getting the height of all blocks in the chunk, i decided to use this method.

It seems that it has a "memory" for when blocks are placed very high up, and does not immediately purge them. So a block could be temporarily placed very high up and it is not until a server restart that it is lowered back down to the actual height.

private int getChunkHeight(final int chunkX, final int chunkZ) {
    ChunkSnapshot chunkSnapshot = this.world.getChunkAt(chunkX,chunkZ).getChunkSnapshot();
    for (int y = 15; y >= 0; y--) {
        if (!chunkSnapshot.isSectionEmpty(y)) {
            return y+1;
        }
    }
    return 16;
}

This code will return 16 if a block was placed at y 255 and then removed