This is a pretty basic problem that happens when you do something rather complicated.

Normally when you merge posts.. the items all line up in the order that they were posted. This is because post the post id and timestamp are sequential.

But what if you copy a topic and then merge it?

What happens is lets say you create a duplicate copy of a topic created a year ago. Then you merge that duplicate topic with a newer topic. You would expect the older posts (by time) to appear first... but since it was a duplicate topic with higher valued Post IDs.. they come up afterwards when viewing the forum. It improperly sets the topic index in the database as starting with the newer topic and not the older duplicate.

Kind of complicated.. but basically the forum is looking for the oldest id, and not the oldest post when putting together the information in the topic database. Specifically "First Post ID" and "First Post Poster".. it doesn't update correctly.