"They can almost always be seen on dive trips to Lembeh"
Used to be the case- but during my last two trips down there, I only saw one and the long-time guides talked about how they're becomming increasingly rare. My experiences not seeing many could have been because of seasonal chances in abundance, but the guides are more likely to notice year-to-year changes that might result from over-collection or trawling disturbance. They certainly aren't a sure-thing on a dive, or even a whole trip of three dives/day for five days straight.