Let consider the following function:

def my_func(k):
    if k == 3:
        return ["A string..."]
    else:
        yield from range(k)

What would be expected behavior, if we called

Instead of exected output ["A string..."], we would get an empty list []!

The same behaviour is true, if we replace yield from with a loop, e.g.

def my_func_loop(k):
    if k == 3:
        return ["A string..."]
    else:
        for j in range(k):
            yield j

When yield from has come (Python 3.3+), it becomes possible to use return (with values) inside generators. The essence of such behavior becomes clear from PEP380. However, even being explainable from PEPs, this issue produces very unintuitive behavior.
Probably, this example could be included to this project. I can prepare a pull request for this.