I couldn't come up with a better title, so let me explain what the bug is.
Let's say we have user 1, user 2 and me (rado84) and I'm a moderator, the other two are just registered users. User 1 creates "Topic A", user 2 creates "Topic B" and both topics are about the same thing, so I decide to merge them into one new topic named "Topic C" which I have created in order to be able to do the merging. Now that "Topic C" is created there is something that sticks out a mile: User 1's post is first in the topic and yet while you're looking at the topics list the forum displays me as an author. My post has become the latest one in "Topic C". That's bug 1.
Bug 2 is this: User 1's post is the first one and therefore I can use only his post to edit the topic and perhaps - pin it. But if I do that "Topic C" gets unmerged and "Topic A" and "Topic B" go back to the state of 2 separate topics that they were before the merging and "Topic C" is being completely erased from the database.
I hope you understand what I mean but if it's necessary I can make a video demonstrating the bug.
This bug can be easily fixed if you make the post of the moderator who created "Topic C" to remain first in the newly created topic and also - if you disable unmerging if "Topic C" has suffered changes, such as labeling the topic as a message or pinning it.
That happens with all phpbb3 versions we've tried so far including the latest one.