I have to be honest, I genuinely don't think there's a single, widely agreed-upon main plot twist for 'Invisible Dragons' because it sounds like a made-up or misremembered title. I've searched through online book databases, fantasy forums, and Goodreads lists and haven't found a novel with that exact name. It could be a case of the 'Mandela Effect' where a bunch of people think they recall a book that doesn't exist.
Sometimes, people conflate titles or concepts from other works. The name might be mixing the 'Invisible' part from something like Ralph Ellison's 'Invisible Man' with the dragon motif from countless fantasy series. If you're thinking of a story about dragons that are magically hidden or perceived differently, you might be recalling a subplot from a larger series like 'Temeraire' or even a video game lore. Without a concrete author or series, pinpointing a twist is impossible. My best guess is someone online used it as a placeholder title in a writing prompt, and the idea just stuck in people's minds.