Not sure what you mean by resetting during preseason.
In Preseason - It's practice, not even common practice, this is just how it is - teams play their starters for the first quarter of the first game / the first half of the second game / the first three quarters of the third game.
Once you reach those points, the depth chart will start your backups. Does it do a good job? No. Does it make good decisions for which backups go where? Absolutely not.
The only real way to deal with this if you care to - is to go in and manually adjust it after the backups sub in for their preseason reps.