Gnus does not find the thread root for some messages

[Christoph Groth]

Hello,

I have configured Gnus threading like this

(setq gnus-summary-make-false-root nil
gnus-summary-thread-gathering-function 'gnus-gather-threads-by-references)

to be a bit stricter on what constitutes a thread. This has been
working perfectly for zillions of messages.

However I noted recently that Gnus fails to identify the root message
for email notifications sent out by a certain "gitlab" server that I
use. I can see that when I set gnus-summary-make-false-root to `dummy'.
Then the various replies are grouped around a dummy root, but the real
root message, while present, is not recognized.

For example the message containing the following headers

Message-ID: <***@gitlab.kwant-project.org>
References: <reply-***@gitlab.kwant-project.org>
Subject: kwant | refer to PropagatingModes docstring from physics.leads.modes (!169)

is not identified correctly as the parent of the message containing the following

Message-ID: <***@gitlab.kwant-project.org>
In-Reply-To: <***@gitlab.kwant-project.org>
References: <***@gitlab.kwant-project.org>
<reply-***@gitlab.kwant-project.org>
Subject: Re: kwant | refer to PropagatingModes docstring from physics.leads.modes (!169)

Isn't this strange? Everything seems to be set correctly.

Gnus' awesome configurability allows me to work around this problem by
setting gnus-summary-make-false-root to `adopt' and
gnus-summary-thread-gathering-function to
`gnus-gather-threads-by-subject' just for this group (using group
parameters). But I would like to understand what's going on, especially
whether this is a bug in Gnus or in gitlab.

A quick unrelated question: Is this the famous ***@gnus.org mailing list, or
a separate one? Is ***@gnus.org still active?

Christoph