Parallax
The apparent shift of nearby objects with changing viewpoint — the main cause of visible seams.
Hold a finger in front of your face and close each eye in turn: the finger jumps, the background barely moves. That is parallax — and exactly what happens with a 360° camera whose lenses sit a few centimetres apart.
During stitching the images must match in their overlapping regions. Distant objects do so easily, near ones do not: they appear offset in the two frames, producing breaks, duplications or blurred edges.
In practice that means keeping your distance. A good metre from walls, furniture and people, and never placing the camera in tight corners or doorframes. What is too close cannot be repaired afterwards.