Why is Django's returning stale cache data?

I have two Django models as shown below, MyModel1 & MyModel2:

class MyModel1(CachingMixin, MPTTModel):
    name = models.CharField(null=False, blank=False, max_length=255)
    objects = CachingManager()

    def __str__(self):
        retu "; ".join(["ID: %s" % self.pk, "name: %s" % self.name, ] )

class MyModel2(CachingMixin, models.Model):
    name = models.CharField(null=False, blank=False, max_length=255)
    model1 = models.ManyToManyField(MyModel1, related_name="MyModel2_MyModel1")
    objects = CachingManager()

    def __str__(self):
        retu "; ".join(["ID: %s" % self.pk, "name: %s" % self.name, ] )

MyModel2 has a ManyToMany field to MyModel1 entitled model1

Now look what happens when I add a new entry to this ManyToMany field. According to Django, it has no effect:

>>> m1 = MyModel1.objects.all()[0]
>>> m2 = MyModel2.objects.all()[0]
>>> m2.model1.all()
[]
>>> m2.model1.add(m1)
>>> m2.model1.all()
[]

Why? It seems definitely like a caching issue because I see that there is a new entry in Database table myapp_mymodel2_mymodel1 for this link between m2 & m1. How should I fix it??