If your stats are not at 30, 50, or 70 in Career
mode, they will begin decaying after a few weeks unless an attribute point is added to them before the yellow bar, which represents how close each stats is to decaying, completely disappears. Keep an eye on your stats, and either leave them at 30, 50, or 70, or make sure you are adding at least one attribute point to them before the yellow bar disappears and they begin decaying each week.
Once you are able to get your created fighter's stats to 100 in Career mode, get all desired stats to 70, 50, or 30 to prevent the stats from decaying below those levels. Then, try to keep storing up as many points as possible from Free Sparring. Save those points until Career mode is close to ending (sometime in 2021). Then, use all the saved attribute points to increase your fighter's stats as desired. Once Career mode ends, you will still be able to use your created fighter in other modes, including online. By using this method, you will not lose stats due to decay, and will have a fighter with high attributes once Career mode ends.