The fix is something like this for _ArgumentGroup, but as the container may not be an _ArgumentGroup, it breaks the tests.

--- Lib/argparse.py
+++ Lib/argparse.py
@@ -1635,9 +1640,13 @@ def __init__(self, container, title=None, description=None, **kwargs):
         self._has_negative_number_optionals = \
             container._has_negative_number_optionals
         self._mutually_exclusive_groups = container._mutually_exclusive_groups
+        self._container = container

     def _add_action(self, action):
-        action = super(_ArgumentGroup, self)._add_action(action)
+        if self.title:
+            action = super(_ArgumentGroup, self)._add_action(action)
+        else:
+            action = self._container._add_action(action)
         self._group_actions.append(action)