Sorry to necrobump a thread, but i'm having the exact same issue and I thought I might as well bump this instead of making a new one.
My problem only seems to be appearing on a number of cars, instead of all of them; my last two traffic stops I checked the plates on an Ingot and a Mesa and Gauntlet and they all showed the correct name, but later that same session I ran the plates on a cement mixer and it came back with a hash instead of a name. It also appears that LSPDFR and Police Radio's plate lookup works for most vehicles, but when I get told about a car by a witness from a callout addon it just gives me a hash instead of a name.
I was wondering if anyone in this thread had solved the issue, it's not a major issue, but its very annoying. The only custom car mod I have installed was the coroner speedo by Smaen and I checked if their were multiple entries for it in my update.rpf and the DLC folders but I found nothing.
Any help would be appreciated.