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Duh, my turn to forget. For any beginners reading this ...
>>> class ms(str): pass
...
>>> a=ms('a')
>>> type(''.join((a,)))
<class '__main__.ms'>

Expanding mhw's second point:

>>> e=ms()
>>> type(e)
<class '__main__.ms'>
>>> import copy
>>> e2=copy.copy(e)
>>> type(e2)
<class '__main__.ms'>
>>> e3=e[:]
>>> type(e3)
<type 'str'>
>>> id(e),id(e2),id(e3)
(9494608, 9009936, 8577440)

so [:] is not exactly an abbreviated synonym for copy().  Is 
this a butg?  (I haven't rechecked the respective docs yet.)

One reason I hesitate to call the OP's original observation a 
bug is that the whole sujbect of operations on subtype 
instances seems not completely baked.  Knowing the result 
types in all cases may require experiments as well as doc 
reading.