My buddy and I had this issue literally last night. My assumption is that when mistlands was released it changed our worlds, which altered anything we hadn't uncovered yet. The seed generator showed exactly where everything was, like you said, and everything we found matched up except yagluth whom was next. We ended up having two places for yagluth, and one wasn't there. The second one was. So I'd see about finding a different one if possible. If there isn't then I'd contact the devs about it. Trying to sail around forever, checking every plains isn't fun.