Only base method of generic called

I have the following "models":

My base class:

public abstract class Search : Model
{
    //Properties ...

    public void ShallowCopy(Search reference)
    {
        base.ShallowCopy(reference);

        //Do stuff
    }
}

My inheriting class:

public class Vehicle : Search
{
    //Properties

    public void ShallowCopy(Vehicle reference)
    {
        base.ShallowCopy(reference);

        //Do stuff
    }
}

My base "viewModel" is generic:

public abstract class MasterDataWithoutAddressViewModel<TPrimaryModel> : MasterDataViewModel<TPrimaryModel> 
    where TPrimaryModel : Search, new()
{
    public void JustAMethod() 
    {
        //do stuff

        foreach (TPrimaryModel primaryModel in primaryModels)
        {
            TPrimaryModel primaryModelCopy = new TPrimaryModel();
            primaryModelCopy.ShallowCopy(primaryModel);
            //Do more stuff
        }
    }
}

My inheriting "viewModel":

public class VehicleViewModel : MasterDataWithoutAddressViewModel<Vehicle>
{
    //...
}

With primaryModelCopy.ShallowCopy(primaryModel); I expected the ShallowCopy of Vehicle to be called. However just the method of the base class Search is called. Debugger shows that primaryModel and primaryModelCopy are both from the correct type (Vehicle).

This is the complete inheritance hierarchy of my "models":