Working on phone, but repro steps appear to be based around this, on 'older' topics...will try to flesh out exacts tomorrow.
1) Start a topic, publish immediately
2) Reply to the topic (you will want to turn off post merging)
3) edit the first post, notice publish fields show actual date/time and not 'Immediately'
4) save edit.
5) first post is now the last post, and vice versa.