The only possible explanation is that the puzzles you complete disappear from the list with uncompleted ones. I don't think they disappear completely, they're just removed from the list to make it more easier for you to see the remaining puzzles available in the game. I've looked on the web for this issues as well, but there isn't something similar which indicates that it's not a problem with the software and quite possibly it's the way it works. Reinstall the game/application and check its behavior, because this could also be a bug.