So a while back I was looking at the Forza Series and I saw Forza Horizon 5 there. Now, I look at the recent games and see a new Forza Horizon 5 That's not the series. What happened to the original leaderboards?

Forza doesn't have a series, so it could have been requested by anyone, which is what happened. It was accepted before the Forza mods were able to request the game.

1. By series I meant all of the games
2. I meant that this game was already on the site around a month ago, and no I see it as it just got added.

Sometimes when leaderboards get accepted, the person who requested them already gave up on speedrunning or whatever and never bother to set up the leaderboard. When that happens the leaderboard just gets deleted, and someone else requests it later. That's probably what happened here.