@MichalRaven
Have you tried to reconnect with the servers after playing offline?
By that, I mean play offline with the backup save that has all your cars.
Perhaps update the console save by doing a quick GT League race.
Then reconnect the console to the internet
without closing the GT App or powering off the console.
And then in GT use the "reconnect" button.
I can't buy cars offline, of course.
But I can race Arcade offline, earn Credits etc, change into different cars, etc., and then reconnect to the GT servers and it updates all my progress and changes to the server save.
I'm wondering if it may recognise the cars in your garage if you try this.
*Edit*
There may be a reasonable chance this could work for you.
I just tried a couple more things.
It's easy for me to play offline as my console is wired, so I simply pull the ethernet cable from out of the back of the console.
I did this and started GT.
I sold a car and earned 100k, and lost a car from my garage while offline.
I changed into another different car.
I then plugged the ethernet cable back into the console and "reconnected" with the GT servers.
The server save was then automatically updated with my new garage car count and extra credits.
Worth a shot IMO.