In version 18w47a, there're problems loading maps from the old versions. The logic's thread is completely locked-up (mob's AI not working, no items dropped when the block is destroyed). When a player is outside the boundary of the loaded chunck, the player will fall down from the edge very slowly (like falling from the cliff, but much slower). If loading a map generated by the new version, even though it does not locked-up, but it loads much slower than previous version. (And I have lost my saved survival map that I have played for a loooooooooooong time!!!)