Lush cave blocks (moss blocks and clay) do not generate directly below decorative blocks like cave vines and glow lichen in the lush caves biome and are replaced with stone or deepslate, depending on where the cave is generated. Does not happen when there is air between the decorative block and the block below. The world generator seems to be treating these decorative blocks as solid blocks, and therefore not generating the appropriate blocks in the spot it thinks has a solid block above.