There's a truly outstanding and ongoing point of Rocket League Items reference for this precise kind of mix. Fortnite, which is presently the best game on earth, joins progress through its own Epic Games account framework.
This permits players to purchase the Battle Pass in one spot and work toward new awards over all stages. Rocket League could possibly be attempting to mimic this model.
(Assuming this is the case, don't be shocked if Sony will not take care of business - simply like how it won't let Fortnite players on PS4 utilize their Cheap Rocket League Items Epic records on different stages.)
