[..] the "view first unread post" functionality
… works for me, been using it since I joined the forum.
Regardless of how many posts per page your settings are, if the number of new posts spills out onto the second page before you've read them all on the first page, then the "view first unread post" will take you to the first post of the second page, instead of the first unread post still on the first page.
Nope, not here. (I was quite tempted to edit out all the “you”s here
But I wonder now, if its because I've already opened the first page, instead of using this newly found link.
I’d bet on this.
I'm quite sure it'll fail for you too, but its difficult to notice and test.
First, you've got to remember the last time you looked at some particular thread. Maybe there were only 8 posts on it and you've read them all the last time you looked at it.
Then the next time to look at the thread, maybe there's now 12 posts and suddenly 2 pages (at the default 10 post per page).
The "view first unread post" link should take me to post #9 (my first unread post, and still on the first page), but it doesn't. Instead, it takes me to post #11 (the first post on the next page), skipping two posts that I haven't yet read.
This only happens when the posts initially spill over onto the second page. Threads longer than 3 or more pages work as expected again.
I first noticed this when I had my settings at 40 posts per page. Less annoying on less active threads.