After the 1st driver crosses the finish line, it should be the last lap for all drivers - as in mostly all real races.
Or at least ghost finished drivers (as in FM4) to not hinder lapped drivers.
Interesting: This is a rule that got invented for the Italian Grand Prix in 1922. The driver on second place was already two laps down (this was quite new due to Monza being a quite short course with “only” 10 miles). When the leader crossed the finish line they decided to give the remaining drivers the checkered flag as well (probably because it makes no sense when the leader already finished and celebration started but the other drivers still do their race).
This basically means that Forza still not implemented a rule that is over 100 years old. And this might be acceptable for a game from 2005 where Forza was new in the business but not when we now have the 8th game of this series. Sometimes it feels like nobody at Turn 10 ever saw a real motor race because what @Mighty_Muelo here proposes is one of the most basic standard rules in motor racing nowadays.