It seems that `.join` methods typically return the type of the separator on which they are called:

    >>> bytearray(b" ").join([b"a", b"b"])
    bytearray(b'a b')
    >>> b" ".join([bytearray(b"a"), bytearray(b"b")])
    b'a b'

This is broken in UserString.join:

    >>> from collections import UserString as US
    >>> x = US(" ").join(["a", "b"])
    >>> type(x)
    <class 'str'>

Furthermore, this method cannot even accept UserStrings from the iterable:

    >>> US(" ").join([US("a"), US("b")])
    Traceback (most recent call last):
    ...
    TypeError: sequence item 0: expected str instance, UserString found.

I can submit a PR to fix this.

It is a duplicate of issue16397.

This is not a duplicate: issue16397 concerned

   " ".join([US("a"), US("b")])

While this is concerned about the return value and acceptable parameters for UserString.join().

You are right. Sorry.

This API is very old and doesn't seem to have a caused any issues in practice.  Changing the API now is more likely to break existing code than to help anyone in the future.