Django: ManyToManyField.limit_choices_to seems not working properly

Question

I have following models:

models.py:

def limit_name_choices():
    return {"pk__gt": Name.objects.last().pk} 

class Name(models.Model):
    name = models.CharField(max_length=100)
    primary = models.BooleanField()


class Robject(models.Model):
    project = models.ForeignKey(to=Project, null=True)
    author = models.ForeignKey(
        to=User, null=True, related_name="robjects_in_which_user_is_author")
    create_by = models.ForeignKey(
        to=User, related_name="robjects_created_by_user", null=True)
    create_date = models.DateTimeField(null=True)
    modify_by = models.ForeignKey(to=User, null=True)
    name = models.ManyToManyField(
        "Name",
        related_name="robject_name",
        help_text='The name of robject',
        limit_choices_to= get_last_name_pk()
    )

views.py:

def robject_create_view(request, *args, **kwargs):
    if request.method == "POST":
        form = RobjectForm(request.POST)
        if form.is_valid():
            return redirect("/")
    else:
        form = RobjectForm()
    return render(request, 'robjects/create_robject.html', {"form":form})

and forms.py:

class RobjectForm(forms.ModelForm):
    class Meta:
        model = Robject
        fields = '__all__'

        # this comes from django-addanother (admin popup functionality)
        widgets = {
            'name': AddAnotherWidgetWrapper(
                forms.SelectMultiple,
                reverse_lazy('add_name', args=['proj_1']),
            ),
        }

I created imitation of admin Robject create form. This means I added 'plus button' next to ModelMultipleChoiceField name field, which leads to popup where I can create new Name. I intend to have empty name field (without choices) every time I open my robject create form. Unfortunately, I see previously created Names when I refresh the form page.

My problem is, function limit_name_choices isn't called every time new form is instantiated. Is it a bug, or I am doing something wrong?

From django documentation: https://docs.djangoproject.com/en/1.11/ref/models/fields/#django.db.models.ForeignKey.limit_choices_to

If a callable is used for limit_choices_to, it will be invoked every time a new form is instantiated. It may also be invoked when a model is validated, for example by management commands or the admin. The admin constructs querysets to validate its form inputs in various edge cases multiple times, so there is a possibility your callable may be invoked several times.