The list of biomes a player has visited, which is presumably supposed to be used for the “Adventuring Time” achievement (but is not because of
MC-30527), is reset on each login.
One way to reproduce:
- Get the “The End?” achievement.
- Leave the End.
- Log out. (Save and quit to title on singleplayer)
- In the server's world file, have a look at stats/<your player name>.json — notice several biomes, including Sky, listed under achievement.exploreAllBiomes, under progress.
- Log back in.
- Notice that the Sky biome is missing from the jsonfile. (And in fact, it is empty until you move again.)