On the common point that the expansion vehicles aren't the best fit for the expansion's actual events, I feel like the same thing happened with the previous expansion too. Hot Wheels, which was all long straights and maximum speeds, included two high downforce track cars, a Pikes Peak buggy, a rallycross car, and a gear-limited drag car.
However, the season directly before the HW expansion had a full set of very appropriate cars, with five Hot Wheels specials and two muscle cars with extreme 1500hp+ engine swaps. The same is true of the latest expansion - Series 18 had a Group B rally car, a rallycross car, a lower-performance trophy truck, two buggies, and a pair of classic off-roaders. There's even duplication of prizes again, with the quattro being given out as part of the expansion career, like the Hot Wheels cars were.
With it happening twice now, it must be a deliberate design decision. Cars appropriate to the expansion are given away free as a taster (and perhaps to tempt players to buy the upcoming expansion), while the expansion itself includes a different set of vehicles as paid content. Doing it this way means that the player can use those appropriate free cars in the expansion, as the paid content is given away as prizes as late as the very last race. The paid vehicles can then be taken back and used in the base game content, for example the COPO Camaro, WRX ARX, and the various Baja trucks all being best suited to event types not featured in the expansions that they debuted in (drag, dirt, and cross-country respectively). This potentially means that any player spike from the new expansion launching continues into base game multiplayer lobbies as people who have finished the expansion try out their new toys.