One test database is not created when two database are used

Question

I have two databases to use in django project.

in settings.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        "NAME": "mydb",
        "USER": "root",
        "PASSWORD": config("DB_PASSWORD"),
        "HOST": "127.0.0.1"
        "PORT": 3306,
        'OPTIONS': {
            'charset': 'utf8mb4',
            'init_command': "SET sql_mode='STRICT_TRANS_TABLES'"
        },
        'TEST': {
            'NAME': 'test_mydb',
            'MIRROR': "default",
        },
    },
    'extern': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': "mydb_extern",
        'USER': "root",
        'PASSWORD': config("DB_EXTERN_PASSWORD"),
        'HOST': "127.0.0.1",
        'PORT': 3306,
    },
    'TEST': {
        'NAME': 'test_mydb_extern',
        'MIRROR': "default",
    },
}

extern has the table 'MyInfo'

in extern_app/test.py

from django.test import TestCase
from extern_db.models import MyInfo
class HelpViewTest(TestCase):
    databases = {"default", "extern"}
    def test_model_script(self):
        
        items = MyInfo.objetcts.all()

        for cnt,i in enumerate(items):
            print(cnt)

There comes error like this.

django.db.utils.ProgrammingError: (1146, "Table 'default.myinfo' doesn't exist")

extern has the myinfo table, however somehow application try to search myinfo from default.

Why it happens?