I can reproduce this, but oddly it only seems to have gone wrong for
that one thread -- all the other threads in the mailbox seem to be
sorted correctly. I also note that the Summary buffer displays the date
of both messages in the thread as one day earlier than the Date header
in the message itself. So I wonder if there's just something weird about
one or both of the messages that's causing Gnus to handle the date
incorrectly.
When I have a bit more time I'll try to figure out what date Gnus is
trying to use for that thread.