Yes, this is very common. We saw the exact same thing at that age. I remember feeling concerned about this but ultimately I shifted my thinking. It wasn't that nothing was getting finished, but that the child got what he needed from the project and was ready to move on. The purpose for him wasn't the end product, but the process of discovery. Also, realistically, a team of adults probably couldn't complete the amount of work these kids are starting on.
One thing about motivated self directed learners is they can be good at putting themselves into that zone of optimal engagement and acquisition of new material or skills. It sounds like that's exactly what she's doing. She's putting herself in the place where she gets the most of out if and she has a sense when she's beyond that it is okay to let stuff go. That's very healthy I think.
And, if it makes you feel better my overly ambitious seven year old has turned into a teenager who gets stuff done. He continues to be good at evaluating if a project, book, or activity is worth his time and he is willing to let stuff go that isn't. Again, that's a healthy and good thing.
