Reverse function fails for a single app

[Derek]

Using Django 2.2 and Python 3.9

When I try to use reverse() for testing one of my apps, it fails. All the others seem to work OK.

An example code snippet:

from django.test import TestCase

class TestCaseSimple(TestCase):

    def setUp(self):
        User.objects.create_superuser(
            username='te...@example.com',
            password='password',
        )

    def test_test(self):
        self.login = self.client.login(username='te...@example.com', password='password')
        assert self.login is True
        url = reverse('admin:appone_modelone_changelist')
        print(url)  # passes
        url = reverse('admin:apptwo_modeltwo_changelist')
        print(url)  # fails

Part of error response:

E       django.urls.exceptions.NoReverseMatch: Reverse for 'apptwo_modeltwo_changelist' not found. 'apptwo_modeltwo_changelist' is not a valid view function or pattern name.

/home/derek/miniconda3/envs/tp32/lib/python3.9/site-packages/django/urls/resolvers.py:677: NoReverseMatch

During normal use, the app behaves fine and any other tests not requiring reverse all work OK. 

What could cause the URLs for this single app not to be found by the resolver code?