Yes, and the issue is more complex than it seems. To give you a quick synopsis, FM4 launched with around 500 cars and by the end of all (reasonably priced) DLC, there were about 670 or so. From FM5's announcement it was touted as having "hundreds of cars" and "the most diverse car list ever". No interviewers ever asked what that meant until Gamescom, where just one (Xboxygen.fr) got the answer that there would be less cars than FM4. But no one really wanted to take that seriously.
At first T10 claimed that they were using "all-new" assets in the making of the game, that all old models had been cut out. Then they said that only models that weren't "AAA quality" were tossed. However, when screens of certain cars began to show up, fans could immediately tell that they were the same models, with the same inaccuracies as before, but with new textures and lighting optimization. When the final car list dropped, people were shocked (except me, I had felt odd about it the whole time). People began to conclude that Forzavista, a feature introduced in FM4 that allowed you to "explore" the car by opening hoods, doors, etcetera, was to blame, since every car had this feature. But now that people are getting their hands on the game, they're finding that not all of them have opening hoods and such...
Even worse is that many of the cars added as DLC to FM4, while they were making FM5 (they like to brag that they started work on FM5 before FM4). These cars were most definitely future-proofed, but a vast majority of them are now gone, including a lot of popular additions (Like the Ascari KZR1, Hudson Hornet, Maserati Biturbo, and Pontiac Fiero).
So if the models are ported over, and they aren't adding these new features they said they would, then they have been lost for no discernible reason, except to be sold back as DLC that is now twice as expensive.
Sorry about the rant there.
EDIT: I have no doubt that GT will be losing cars when it's going to next-gen... but they're in a different situation. A lot of Forza's car models were not two generations old, they would have worked fine with new textures (like only some of them got).