Filing this myself since I'm sure it will get reported When moving a topic into another forum, even if you've read some of the replies in the topic it may mark some of them as being new.

Working on this fix.

After working on this, I realize it's one of those things that can't be fixed. Posts are marked read/unread on a per forum basis since your last visit. Since moved posts are actually new in that forum, it works as expected.