1) Replies don't edit the topic's Subject in the web UI. Only the Edit Topic function does that.
2) When Groups.io processes a message posted by email I don't think it does any modification to the subject line as emailed out to members other than to ensure that it includes the group's Subject tag. In particular it doesn't re-write the message Subject to match the Subject of the topic (if any) to which that reply is attached in the web UI.
