What can cause @Primary to not have its intended effect?

Question

In a Spring Boot app, I have the following:

public interface Car {
    ⋮
}

@Component
public class OldCar implements Car {
    ⋮
}

@Component
@Primary
public class NewCar implements Car {
    ⋮
}

@Component
public class Driver {

    @Autowired
    private Car theCar;

    ⋮

}

(Note: Names of classes have been changed to protect company secrets. :-)

When I launch this app, it bombs with a NoUniqueBeanDefinitionException, specifying:

Could not autowire field ... theCar; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException No qualifying bean of type [Car] is defined: expected single matching bean but found 2: newCar,oldCar

However, if I modify the type of theCar to either OldCar or NewCar (to eliminate the ambiguity), everything works fine.

It seems to me that @Primary is not having its intended effect. I am looking for ideas about what I can examine in order to discover potential causes of this unexpected behavior.

What could cause this? Why is @Primary not working?