In some world seeds, when in the old world generation, the seeds in the seed picker generate the right features, like Stronghold below, and survival island. But when created with the Caves and Cliffs experimental toggle, they don't generate the correct features, like the Stronghold village not generating in front of the player, or the island not generating with you on it. The generation seeds need to generate with respects to the old world gen. The picture on the left is the old Stronghold below, and the one on the right is the new generation.