While playing along with the cubic chunks mod, which is used to extend the Y at almost an infinite limit using chunks that are cubes, after generating a new world, player can't get over a certain Y or under a certain Y because chunks don't get generated when the player moves. The only chunks where you can move are the ones generated between 2 Y values (it still generates chunks across X and Z). The player model gets freezed when you go past that Y level, and there is no error in the log.