Imitating Jane

It's like this. A previously unknown manuscript of some long-dead author is discovered. Let's just pick someone at random - oh, I don't know... Jane Austen. The manuscript contains the text of a complete novel, Hope and Hospitality, and it doesn't take people long to ascertain that this is another outstanding work. It's up there with Pride and Prejudice and Emma. The authorship is Jane's, no question about it. All the experts concur; computer analysis says no different.

And then... It turns out that the manuscript is a fake. This is revealed, the opinion of the experts and the computer analysis notwithstanding, thanks to the efforts of an investigative blogger. She looks into the circumstances of the manuscript's discovery, and finds there enough evidence of the fraud to sink a battleship.

Is Hope and Hospitality still the great work people at first took it for, and if not why not? Note that the question isn't whether the (real) author of the work is the genius Jane Austen was. She isn't (let's say), because she's imitating. But is the novel as good? Remember that it was in the class of Pride and Prejudice and Emma - ex hypothesi, as it is said. So if not any longer, then why not?

[W]hy does our estimation of a work of art change when we discover it is a fake?