This stems from a query on StackOverflow: https://stackoverflow.com/questions/47272460/python-tuple-unpacking-in-return-statement/

Specifically, the following syntax is allowed:
def f():
    rest = (4, 5, 6)
    t = 1, 2, 3, *rest

While the following result in SyntaxError:
def g():
    rest = (4, 5, 6)
    return 1, 2, 3, *rest

def h():
    rest = (4, 5, 6)
    yield 1, 2, 3, *rest

Looking at the original commit that enabled tuple unpacking in assignment statements:
https://github.com/python/cpython/commit/4905e80c3d2f6abb613d212f0313d1dfe09475dc

I don't believe this difference is intentional.

My GitHub repo incorporates a fix for this; I'll file a pull request momentarily.