>>> Part-1: There is a bug somewhere in the merge process ...
> This should be fixed now.
Yes it definitely is, clicking on Reply prefills the correct subject now, thanks!
> When replying via email to a message that has been merged into a different topic (with a different subject),
> we automatically rewrite the incoming reply's subject line to match the merged-into topic subject,
> and we should thread it properly into the merged-into topic. I just tested it and it worked for me.
Works for you?? Doggone it, I now had to tax my grey matter again, lol, but seriously, your reply made me setup a test case in a test group and I was able to replicate both your successful and my unsuccessful threading outcomes.
Turns out this Part2 threading problem is not merge-related, it's split-related. Change merged into split in your statement above, and now an email-reply to a message which used to belong to TopicA-subject (but has been split into TopicB-subject) and hence still has the original TopicA-subject in the reply email, eludes detection and no subject-rewriting takes place, no subject-matching takes place (even if there exists an active TopicA-subject in the archive), and instead creates a new "clone" TopicA-subject. So in merged threads it does what you stated but in split threads it behaves differently.
I just emailed you both the real-world and test-case details to the support address.
Thanks and Cheers,