It makes no sense that tuning doesn’t change the PI if PI is simply a test of your car’s performance on a virtual test track.
If you intentionally destroy your car with a bad tune to where it tops out at 60 MPH and has insane jelly roll suspension, it should not retain its previous PI as it would clearly not go around the PI test track as fast.
It would only make sense to retest the car WITH the new tune and see how that changes the PI.
This also help new tuners figure out what actually makes their car perform better as well.