Unable to overwrite init value of a function once set by monkeypatch for singleton class

I have a very simple python class with a constructor:

from utils.util import Singleton
class VaultAuth(object):
    __metaclass__ = Singleton
    def __init__(self, prefix_path, address):     

        self.path = prefix_path
        self.vault_url = address            
        self.is_authenticated = False

    def get_secrets(self, region):         
        print self.is_authenticated 
        if not self.is_authenticated:
            raise RuntimeError("Failed to fetch secrets")
        else:
            return True

where Singleton class looks as follows:

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

To write unit tests I have:

@pytest.mark.unit_test
def test_get_secrets(monkeypatch):
    def mock_init_auth_false(self, *args, **kwargs):
        self.path = "dummy_path"
        self.vault_url = "dummy_url"
        self.is_authenticated = False

    def mock_init_auth_true(self, *args, **kwargs):
        self.path = "dummy_path"
        self.vault_url = "dummy_url"
        self.is_authenticated = True

    # Negative case - auth is false
    monkeypatch.setattr(vault1.VaultAuth, "__init__", mock_init_auth_false)
    secrets_manager = vault1.VaultAuth(prefix_path="prefix", address="https://vault")
    with pytest.raises(RuntimeError) as exception:
        secret_data = secrets_manager.get_secrets(region="test_region")
    assert "Failed to fetch secrets" in str(exception.value)
    monkeypatch.undo()

    # Positive case - auth is true
    monkeypatch.setattr(vault1.VaultAuth, "__init__", mock_init_auth_true)
    secrets_manager = vault1.VaultAuth(prefix_path="prefix", address="https://vault")
    assert secrets_manager.get_secrets(region="test_region")   

The first test print is_authenticated value as False as per expectation but the second test also prints it as False. If I reverse the order of the tests both print True. Any pointers? The class is singleton. How to test a singleton class function?

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