Pre-anvil worlds which have been converted up to 1.13 world format are having issues where their map chunks are being seen as new map chunks. This can be tested by taking a pre-anvil world that has map chunks outside of the spawn chunks. Set the gamerule, "spectatorsGenerateChunks" to false and switch to spectator mode. Fly to the edge of the world where you know there is generated terrain. Then switch "spectatorsGenerateChunks" back to true. This is probably a minor issue but is obviously a bigger issue with server plugins.