OK I suck and I'm sorry, I thought I had replied to you all this time but I never submitted as I probably accidentally closed the reply window and never looked back to check... so here it goes again.
The next unread post feature works great in the tree view, however implementing this in the threaded/flat view was a tricky one and I'll admit is not perfect at all.
You see, when you open a thread in the threaded/flat view, it is assumed that how ever many post are displayed, the reader will scroll down and read through all the post that is exposed in the thread--which will make all messages marked read once this page is refreshed. And this is where the confusion and problem exists.
The next unread is most useful/intended for threads that have multiple pages of unread posts littered throughout the thread so when you land on the first post (root post) you click the unread link it jumps to the next unread message on the next page. However the issue stemming from this (what you are experiencing Ray) is because clicking on the unread link while it does take you to the next unread post, it also refreshes the page which also automatically marks all posts on that page read--therefore killing the next unread links there after even you have not read some of the posts. In the tree view, you don't have this issue because the tree doesn't expose all the posts as you have to click on each individual post to read which will then mark it read.
Are still with me? I'm getting confused even trying to explain it
so I hope this makes a bit of sense.
The good news is that the engineer has some time to revisit this feature and trying to make it work as intended--allowing you to jump through all unread post even if the page gets refreshed in the threaded/flat views... so cross your fingers as I'm hoping to get this issue worked out shortly.
Thanks!
-Lee