I get this problem a lot and wish Evernote would resolve it. I try to minimise the risk by frequently saving the sketch and I also make a note of the sketch version each time I reopen it, so I can keep track. But that only helps if the sketch shows all the different versions on the same note, which often happens; at least then you can choose the correct one. It doesn't help if you lose the sketch completely. It's a real shame because the integration of the pencil, sketch and Evernote is a fantastic idea in concept especially if, like me, you create a lot of hand-written mindmaps. I persist because it is so useful. But I wish I could rely on it more than is currently possible. Unfortunately the Evernote support folk just say it's a unique problem to me that they can't replicate. It's good to know that this is not in fact true and that others suffer the same problem.