gnus-fetch-old-headers = some, but no old headers fetched?

[Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai Großjohann)]

With (setq gnus-fetch-old-headers 'some), I enter the Gnus mailing
list and see the following thread:

EN 13-Aug,  3k: Daniel Pittman  | Re: MML menu in message-mode is poorly named

I then M-: (setq gnus-fetch-old-headers t) RET, mark the message as
unread, re-enter the group, and see this thread:

E +03-Aug,  2k: -> ding@gnus.or | > MML menu in message-mode is poorly named
E  10-Aug,  2k: Per Abrahamsen  | \-> 
E  12-Aug,  2k: Simon Josefsson |   \-> 
E  12-Aug,  2k: Nevin Kapur     |     \-> 
EA 12-Aug,  3k: Andreas Fuchs   |       \-> 
E +12-Aug,  3k: -> ding@gnus.or |         \-> 
   13-Aug,  3k: Daniel Pittman  |           \-> 

Now I go back with M-: (setq gnus-fetch-old-headers 'some) RET,
re-enter the group, and:

   13-Aug,  3k: Daniel Pittman  | Re: MML menu in message-mode is poorly named

I thought that gnus-fetch-old-headers = some is supposed to fetch the
old messages from threads, too?  Is this wrong thinking on my part, or
is Gnus doing something wrong?

(I've observed this behavior for years now.)

kai
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