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I don't think this has anything to do with namedtuple; it's true whenever you create a class in an inner scope (rather than at module level). This is by design, and these restrictions are documented: https://docs.python.org/3.6/library/pickle.html#what-can-be-pickled-and-unpickled
For example, running this script:
 import pickle
 def my_func():
 class A:
 pass
 a = A()
 return a
 a = my_func()
 pickle.dumps(a)
produces:
 Traceback (most recent call last):
 File "test.py", line 11, in <module>
 pickle.dumps(a)
 AttributeError: Can't pickle local object 'my_func.<locals>.A'