I was trying to move some of the messages of one topic into another one as they had gone off-topic.
Part-1: There is a bug somewhere in the merge process that causes merged messages to remember their previous (pre-merge topic) subject and use that to pre-fill the subject field when replying online.
To replicate this: (happened in an unmoderated group)
1. Split let's say half of the messages in a topic (eg. TopicA) into another one, let's say TempTopic
2. Merge the new TempTopic into another one, let's say TopicB
3. Go now into TopicB. If you online-reply to any of the original already-existing TopicB messages, the subject gets correctly pre-filled with Re:TopicB.
4. But if you online-reply to any of the merged messages from the old TempTopic, the subject gets incorrectly pre-filled with Re:TempTopic instead of the correct Re:TopicB. Then for this case:
This should be fixed now.
Part-2: Another weird thing also happened with this topic merge so I included it in here even if it may not be directly related. It broke threading, and it could be a side-effect from a recent fix.
Namely, one used email to reply to one of the original TopicA messages which by now had gotten moved (through the merge) to TopicB. Their emailed reply of course still has TopicA in the subject. You'd think it would thread under TopicA due to having the same subject and stop there, but maybe because the reply message's InReplyTo value is referencing a message which now belongs to a different topic and therefore subjects no longer match between those two, something didn't work right in threading so it just created a brand new topic, with the same exact TopicA title.
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. Off-list, if you send me the group name, the subject of the message, the sender's email address, and when this happened, I can investigate and try to find out why it didn't work in your case.