How can I earn "Car Collector" achievement in Rocket League?

"Car Collector" achievement says that you need to unlock 5 cars to get it. If I recall correctly I started with 3 initial cars and I’ve already unlocked more than 5, here is my garage with 13 cars:

I’ve tried to follow this guide – I finished the Season Championship and then got 1 win for all of my cars in Exhibition mode, but still cannot unlock this achievement or get more cars. Looks like something was changed in recent F2P update.

How can I unlock this achievement now?

I play on PS4.