Not a bug, when it's clearly designed that way. The Nissan is the same car everyone gets. But any parts you win cannot be applied to the supplied car, hence the need for a scene showing you arrive in one of your own. There are a few races like that. For the purposes of scoring, it really is a street spec race. View it as a chance to win parts for any street spec car you own, irrespective of level. You absolutely could not be in a fullstock 8C, as pretty as it is, because it can't win any parts.
As far as the perf spec tuning goes, it might not be to everyone's taste, but in the real world the Ferrari 599XX is a thing. That exists. (Granted, never with iridescent paint and flames.) And now, so does the FXXK, and there will be an F12XX. So if Ferrari aren't above doing that, then modifying a collection of pixels in the shape of an exotic doesn't worry me too much. I stay away from the eye-watering paint jobs though. My perf-spec silver Miura with red interior is very pretty. And in the unlikely event someone says I've ruined it (I haven't - it's very pretty), I can point out that the fullstock is still there.