Have you been merging using the RC?It was a clean car, neither dupe or frozen-money bought.
Maybe you merged over cars with duplicate plates.
For some strange reason my custom plates are considered dirty on some of my cars. My custom plates are in the car but when I go to the mod shop with those particular cars there aren’t any custom plate options. Strangely enough when I then re-merged those cars with my RC that has my custom plates they were cleaned and the options for my plates came back. Is it possible anything like that is going on with your cars?