Hey there, I was having the same problem and did some fiddling with ini settings and I think I fixed the problem. The problem seems to have to do with a conflict of stop the peds document issue probabilities and callout interfaces ALPR probabilities. What I did was I went into my GTA 5/plugins/LSPDFR and opened the stop the ped ini. In there I put all of these settings to zero:
VehicleRegistrationExpiredProbability=0
VehicleNoRegistrationProbability=0
VehicleInsuranceExpiredProbability=0
VehicleNoInsuranceProbability=0
After that, every time I got a ALPR hit on a vehichle and stopped it. It was the owner of the vehicle driving it. Hope this helps you.