I can't tell from the photo, so: does it appear to be original, that is, done at the factory? I rather doubt it. My suspicion is that whatever the factory did, in intending to keep the two parts of the post together, it has failed. Rotation is a symptom of the problem, but is not the actual problem. The through-bolt solves the symptom, but not the problem. I'm not sure the post is safe to ride.