I remember we discussed this on Discord, and you sent me the car. This bug only affects the legacy renderer. There is no fix planned, so the recommended solution is to enable shaders for such cars.
In fact the fading animation is not perfect even in the shader version (i.e., it becomes fully opaque before fading out). The fading animation should be made to preserve the existing opacity, but doing it in a way that remains compatible with other cars will take a bit more work...