So this seems to only happen in the garage immediately after purchasing a new vehicle and not in the pre-race lobbies.
I’ve tested a couple of cars and every time I go into the Aero and Appearance section to fit parts the game won’t load the new body part, it will either toggle the stock and not there (such as spoiler and no spoiler) or it will just stay as stock as you scroll through and even after purchasing the different pieces. Stats wise the parts still effect the car but just nothing visually happens.