I see the point about getting another shops opinion, but in my book, if the shop is taking responsibility, let them take care of the issue. At my shop, we have issues time to time, defective parts, other concerns, missed diagnosis etc... not often mind you, but it does happen. I make sure to take care of any issues because my reputation is the most important thing I have. Give the machine shop that chance. And don’t ride them, let them do their homework and make smart decisions. Good luck, and let us know how it progresses.