There a problem with the total count of topic in a forum when you move topic letting shadow topic in source forum.
In a fresh phpBB install without any topic. (remove welcome topic)
In the forum A write two topics: topic A and topic B.
Via the Mass moderation per forum(mcp.php?f=FORUM_ID_A&i=main&mode=forum_view), move Topic A and topic B to the Forum B (with shadow topic)
Go to the Mass moderation Forum B (mcp.php?f=FORUM_ID_B&i=main&mode=forum_view)
move Topic A and topic B to the Forum A (with shadow topic)
1-Forum A has now 4 topics (from index)
2-Forum B has now 2 topics.
First result part is not really normal, we have only two topic in forum A... and not 4 as you can see from the index.
Second result part is weird too, unless if shadow topics are considered as real topic, so sorry for mistake, but let me doubt even...
I can't provide a patch/fix for now.