The first thing I would check is where the subject crossed the camera. A 360 camera has to combine two fisheye images, so a person moving very close to the lens boundary is a much harder case than someone farther away.

Try reframing the same moment from several directions. If the defect stays attached to the same physical stitch region regardless of your virtual camera, it points back toward capture/stitching rather than the flat export itself.

That seems to be what I have. The rider passes less than half a meter from the camera and the artifact follows that area of the sphere.

Would you run stitching calibration before every important shoot, or only when something looks wrong? I have not changed lenses or dropped the camera, so I assumed calibration was permanent.

I would not treat calibration as a substitute for good camera placement. Calibrate if the camera or software recommends it, but also plan the shot so the most important subject is not crossing the seam at minimum distance.

For editing, keep the original 360 source until the final composition is approved. In VSDC or another editor that lets you reframe later, choose the virtual angle that keeps the critical subject away from the weak stitch region instead of baking a flat intermediate too early.

I also test the export before blaming the source. Render a short section at the final delivery resolution and compare it with the editor preview at the same view angle. Preview scaling can exaggerate or hide small stitching differences.

If the seam is only ugly in one extreme crop, that is different from a global stitching failure across the sphere.

I moved the virtual camera slightly and the problem became much less obvious, but the real lesson is that the subject was simply too close to the seam.

For the next ride I will point one lens more directly toward the action and keep a little more distance instead of assuming I can solve every angle in post.