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:

1. Get the “The End?” achievement.
2. Leave the End.
3. Log out. (Save and quit to title on singleplayer)
4. 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.
5. Log back in.
6. Notice that the Sky biome is missing from the jsonfile. (And in fact, it is empty until you move again.)