Why aren't auto-generated inline objects created in django?

Question

Django

The code:

class FilterPaymentFormSet(BaseInlineFormSet):

    def __init__(self, *args, **kwargs):
        dates = [date.today(), ]
        second_payment_date = next_nth_day_of_month(the_date=dates[0], n=10, include_today=False)
        dates.append(second_payment_date)
        while len(dates) < 22:
            dates.append(next_nth_day_of_month(the_date=dates[-1], n=10, include_today=False))
        initial_values = list()
        for k in range(20):
            entry = dict()
            entry['payment_date'] = dates[k]
            if not k:
                entry['total'] = 4500
                entry['paid'] = True
            else:
                entry['total'] = 3500
            initial_values.append(entry)
        kwargs['initial'] = initial_values
        super(FilterPaymentFormSet, self).__init__(*args, **kwargs)


class FilterPaymentInline(admin.TabularInline):
    model = FilterPayment

    def get_extra(self, request, obj=None, **kwargs):
        extra = 21
        if obj:
            return 0
        return extra
    formset = FilterPaymentFormSet
    ordering = ('payment_date', )

When I click "create a new object" - 21 inlines are immediately created with the correct data. But when I save the object and go into it again, everything disappears, the inline becomes empty. What's the matter?