How to do 2-way data binding using Python+PyGObject's GObject.bind_property function?

The background to this question (and my overall goal) is to structure a Python GTK application in a nice way. I am trying to bind widget properties to model properties using GTK's bidirectional data bindings.

My expectation is that the bidirectional binding should keep two properties in sync. I find instead that changes propagate in one direction only, even though I am using the GObject.BindingFlags.BIDIRECTIONAL flag. I created the following minimal example and the failing test case test_widget_syncs_to_model to illustrate the problem. Note that in a more realistic example, the model object could be an instance of Gtk.Application and the widget object could be an instance of Gtk.Entry.

import gi

gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, GObject
import unittest


class Obj(GObject.Object):
    """A very simple GObject with a `txt` property."""

    name = "default"
    txt = GObject.Property(type=str, default="default")

    def __init__(self, name, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.name = name
        self.connect("notify", self.log)

    def log(self, source, parameter_name):
        print(
            f"The '{self.name}' object received a notify event, "
            f"its txt now is '{self.txt}'."
        )


class TestBindings(unittest.TestCase):
    def setUp(self):
        """Sets up a bidirectional binding between a model and a widget"""
        print(f"\n\n{self.id()}")
        self.model = Obj("model")
        self.widget = Obj("widget")
        self.model.bind_property(
            "txt", self.widget, "txt", flags=GObject.BindingFlags.BIDIRECTIONAL
        )

    @unittest.skip("suceeds")
    def test_properties_are_found(self):
        """Verifies that the `txt` properties are correctly set up."""
        for obj in [self.model, self.widget]:
            self.assertIsNotNone(obj.find_property("txt"))

    @unittest.skip("suceeds")
    def test_model_syncs_to_widget(self, data="hello"):
        """Verifies that model changes propagate to the widget"""
        self.model.txt = data
        self.assertEqual(self.widget.txt, data)

    def test_widget_syncs_to_model(self, data="world"):
        """Verifies that widget changes propagate back into the model"""
        self.widget.txt = data
        self.assertEqual(self.widget.txt, data)  # SUCCEEDS
        self.assertEqual(self.model.txt, data)  # FAILS


if __name__ == "__main__":
    unittest.main()

The above program outputs:

ssF
======================================================================
FAIL: test_widget_syncs_to_model (__main__.TestBindings)
Verifies that widget changes propagate back into the model
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jh/.config/JetBrains/PyCharmCE2021.1/scratches/scratch_14.py", line 52, in test_widget_syncs_to_model
    self.assertEqual(self.model.txt, data)  # FAILS
AssertionError: 'default' != 'world'
- default
+ world


----------------------------------------------------------------------
Ran 3 tests in 0.001s

FAILED (failures=1, skipped=2)


__main__.TestBindings.test_widget_syncs_to_model
The 'widget' object received a notify event, its txt now is 'world'.

Process finished with exit code 1

My specific question is, how can I get the bidirectional data bindings to work?... I would be glad if someone could fix my example or provide another working example.

In a broader sense, are bidirectional bindings the way to go for syncing UI state and model state in a well-structured Python GTK application? What is the intended and well-supported way to do this? thanks!