https://docs.python.org/3/howto/sorting.html#odd-and-ends gives the following example for reverse sort stability:

>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)]
>>> assert sorted(data, reverse=True) == list(reversed(sorted(reversed(data))))

But here all the keys are different, so the result would be the same even if the sort algorithm weren't stable.

You probably wanted to pass to key=itemgetter(0) to both sorted() calls.