Is it unlawful for your car to be repossessed if you pay all that is due after the car is in repo status?

Perhaps it is. That's a maybe. Some loan contracts have a clause in them that defaults the car to the lender in the event the contract is breached. In these cases, the lender will pick up the car as soon as the borrower becomes delinquent in payments. Many of these lenders will not return the vehicle either, preferring rather to sell the car at auction, and collect the remaining balance through other means.