The rating 0 was wrong. A player cannot get 0 for having played less than 20 min.
One explanation for your substitute got an 8 is that he did well for the 18 min, while some others who played the whole match got a 7 because they were rated for their performance during the whole match.
If the player did not do enough in the 18 min for a rating of 8, then there is something wrong with the new rating system.