My main mythic + score for the previous season is no longer showing, but my current season main score is

Once your current mythic + season score has reached 90% of your previous season score, your current season score will completely replace you last season score on the addon tooltip.

For example, if your main character's previous season score was 1000, once you hit 900 score in a new mythic + season, that will completely replace your last season score as your main score.