Updates to tickets cause an email to be sent from redmine@postgresql.org, but replies to "redmine" are not processed as updates to the tickets.
This is correct. The system has never been set up to do that.
It appears this never worked, judging from mails regarding issue #5645, which were sent to "redmine" but not visible in the ticket.
Correct. Never did and was never intended to do.
However, in the past, messages "from" redmine included other parties in "Cc", so my replies also went to Devrim. At least they weren't lost, which is what's happened at 6855.
It appears that soemtime between Feb 2020-June 2020, redmine stopped including other parties in "cc".
Processing incoming mail isn't essential, but the absence of anything else in "cc" led me to believe that redmine would process the mail and at least update the ticket, and/or forward the update to other parties.
This seems to mostly match up with an upgrade to redmine, so it may be the new default in redmine. I don't *think* that was an intentional change at least, and I would have no idea how to change it in the first place :)