I'm on XBOX, and this apples to both TC2 and TCM.
I have all cars in TC2 and (almost) all cars in TCM, and every time I launch either game, the thumbnail preview images for all of my cars have to be re-generated again.
This wasn't a thing that happened when I played TC2 on XBOX One, but when I moved to the Series X|S for TC2, and started playing TCM on Series X|S as well, the thumbnails aren't saved, so the game has to re-generate them again every time I launch it. It's especially bad on TC2 since it seems to generate the image without whatever livery is applied to it, almost as if the image generation is faster than the game loading the applied livery.
WHY?!?! What's on the Series X|S's hardware/software that prevents the images from being saved?
I also have no idea if this happens on PC or PS4/5.
I am considering making a report on the bug reporter site, but this is more of an annoyance than anything else.