I have the same issue with my slicktop CVPI on "policeold1" and "policeold2" slots. I tried using trainer to change paint type and it worked. I think these two slots are by default set to matte, so when you change paint type and color to metallic or classic everything should be fine. I'm still figuring out how to change the paint type in game files to make other officers use proper paint on their cars.
Edit: If you want to change it permamently, go to OPENIV (Grand Theft Auto V\mods\update\update.rpf\x64\data) and edit "carvariatons.meta". Find policeold1 model and edit lines shown on screenshot.