Very strange, considering when I test it on my main account I get different results.
I've tested it multiple times on 2 accounts and get different results.
Consistently different.
On my main account it holds the car it shows on my USB save.
On my alternate account it doesn't hold the car it shows on my USB save.
By that I mean this.
When I overwrite the System save with the USB save, the game still loads me in the WRX.
And I have confirmed the file copy before starting the game.
On the other hand, using exactly the same steps on my main account, it reverts back to the car shown on the USB save.
Both accounts on the same console.
So both on PS4 Ver. 5.55 and GTS Ver. 01.20.9317788E
The only difference that stands out to me is that the main account is a day one account with around 150 days play, the alternative account is (was) around 3 months old with around 25 days play.
Go figure.