Add note on Django TestCase impl to Isolation chapter

As suggested by @rodrigondec in #114.

docs/isolation.rst

@@ -4,8 +4,8 @@ Test Isolation
 Database transactions per scenario
 ----------------------------------
 
-Each scenario is run inside a database transaction, just like your
-regular TestCases.  So you can do something like:
+Each scenario is run inside a database transaction, just like Django does
+it with the related TestCase implementation.  So you can do something like:
 
 .. code-block:: python
 
@@ -15,3 +15,21 @@ regular TestCases.  So you can do something like:
         User.objects.create_user(username=username, password='correcthorsebatterystaple')
 
 And you don’t have to clean the database yourself.
+
+Troubleshooting
+---------------
+
+.. note::
+
+    Users have reported that test isolation and loading fixtures works
+    differently with different test runners (e.g. when the ``--simple``
+    option is used).  This is likely related to the Django TestCase class
+    configured for the runner.
+
+    Be sure you understand which Django TestCase class is used in your
+    case, and how it's implemented in Django. See the `runner module`_
+    and Django's documentation on `Writing and running tests`_ for details.
+
+
+.. _runner module: https://github.com/behave/behave-django/blob/main/behave_django/runner.py
+.. _Writing and running tests: https://docs.djangoproject.com/en/stable/topics/testing/overview/