Xamarin Form passing parameters between page keep a hard link to it - xamarin.forms

Question

I try the new Shell of Xamarin Form 4 for a small project.
I have a list of order, then someone chooses an order and start picking some inventory for this order with barcode. To be simple, I use 2 views and 2 viewmodel.
The process is:
1. User select an order the click a button "pickup"
2. I use ViewModelLocator (TinyIoc) to resolve the correct ViewModel of the pickup view
3. I call Initialize on the new ViewModel and pass some parameters needed. Like a list of items needed to be pickup.
4. Open the view in modal state.
I don't understand that if I change some qty in the list I pass on the second viewmodel, then hit the back button (modal view close). The quantity changed is now in the original viewmodel. How this is possible? I was thinking that passing parameter to a function do not share the same variable but just copy it??
Viewmodel of the first view (look at the Initialize function the List passed and the JobEnCours object)
private async Task GoPickup()
{
Device.BeginInvokeOnMainThread(async () =>
{
if (this.CodeJob != null && this.CodeJob != "")
{
this.IsBusy = true;
PrepChariotSP3ViewModel vm = ViewModelLocator.Resolve<PrepChariotSP3ViewModel>();
await vm.InitializeAsync(this._jobEnCours, this.ComposantesPick.ToList()) ;
await Shell.Current.Navigation.PushAsync(new PrepChariotSP3Page(vm));
this.IsBusy = false;
}
});
}
the the Initialize code on the second Viewmodel (look I set the JobEnCours.Description=Test)
public async Task InitialiseAsync(Job jobEnCours, List<ComposantePick> composantePick)
{
Title = "Ceuillette: " + jobEnCours.CodeProd;
this._jobEnCours = jobEnCours;
this.ComposantesPick = new ItemsChangeObservableCollection<ComposantePick>();
foreach(ComposantePick c in composantePick)
{
this.ComposantesPick.Add(c);
}
jobEnCours.Description = "test";
So, If I do the back button, then in the first VM the JobEnCours.Description is now set to "test" how this is possible?!
Any idea?