During testing, Manager isn't available

Question

I've swapped the User model with my own custom User model. It uses the standard UserManager. I also have a CustomUserChangeForm which is used in my UserAdmin. I've set AUTH_USER_MODEL = "accounts.User" in settings.

This all works fine when using the site and admin site normally through the browser. Tests on my views that use the custom User model also work fine.

Only when I try to run a test directly on the CustomUserChangeForm I get the error AttributeError: Manager isn't available; 'auth.User' has been swapped for 'accounts.User'.

What's going wrong here?

The test:

from .forms import CustomUserChangeForm
from .models import User

    def test_admin_user_change_form(self):
        self.register_user()
        self.register_user("email2@site.com", "username2")
        user = User.objects.get(username="username")
        # Test valid change
        form = CustomUserChangeForm({ "username": "username3", "email": "email3@site.com" }, user)
        self.assertTrue(form.is_valid())
        # Test case insensitive username change
        form = CustomUserChangeForm({ "username": "Username2" }, user)
        self.assertFalse(form.is_valid())
        # Test case insensitive email change
        form = CustomUserChangeForm({ "email": "Email2@site.com" }, user)
        self.assertFalse(form.is_valid())

The form:

from .models import User

class CustomUserChangeForm(UserChangeForm):
    def clean_username(self):
        username = self.cleaned_data.get("username")
        if username and self._meta.model.objects.exclude(id=self.instance.id).filter(username__iexact=username).exists():
            self._update_errors(ValidationError({ "username": self.instance.unique_error_message(self._meta.model, ["username"]) }))
        else:
            return username
        
    def clean_email(self):
        return self.cleaned_data.get("email").lower()

Admin:

from django.contrib.auth.admin import UserAdmin
from .models import User

@admin.register(User)
class UserAdmin(UserAdmin):
# ...
    form = CustomUserChangeForm
# ...

The answers at this question and this question do not seem to apply/work.

Not sure why this was closed. The linked question is different in that everything is working fine during normal use. Only in the automated test am I getting an error. None of the answers provided in that question solve my problem.