I'm sure that pressing space for 'next unread' has usefully gone around and stepped through from the start, if you get to the end of the unreads in a sub but more have autoloaded above your point in the list.
Twice lately I've had one unread showing and 'next unread' didn't jump to it, I had to click away from the sub and back, resetting the selection point to the top. This time I checked where the unread was, and it turned out to be above me in the same thread. I've tested it just by marking three posts unread, selecting the second, tap Next Unread and it goes to the third, tap Next Unread and it doesn't jump to the first:
Whereas If I mark a some posts unread like so: 
and press "next unread" three times from the shown selection, it'll go through all the unreads including jumping upwards.
Not sure if the it's always been like this and I just happened to hit two unreads above me in the same thread lately.