OK, have things working as we wanted. As I assumed, I also needed to adjust the email templates used for PMs to include %{optional_pm}.
Follow up question: At the time this first came up, it was because users who posted a PM to our support group were seeing subject prefixes in reply email notifications that made them worry their messages weren’t private (“PM” wasn’t obvious enough to them).
However, now when I test with non-support users, their email notification subjects don’t include any prefixes at all - just the message title. Those of us who are members of the support group have the group name etc in the email subjects for the same messages. Can you guess why this might have changed in recent months? We don’t recall making a change that would remove all prefixes from the email subjects for non-support users.
I wasn’t involved in the original discussion with users who were worried about messages appearing non-private. It’s possible people were forwarding notification replies via email without going through Discourse.
Thanks.