My spine shifted in the KDP previewer. Is the file wrong?

If your spine text looks off-center or shifted in KDP's previewer, the usual cause is a spine width that no longer matches your page count — for example, your interior grew or shrank after you made the cover, so the spine is now too narrow or too wide and everything slides.

Fix it by setting the Cover Maker's page count to your final interior page count and re-exporting. See Can I change the page count after exporting?

A tiny amount of shift is normal — print-on-demand has a small manufacturing tolerance, which is exactly why you keep important spine text within the safe area and don't run it edge-to-edge. If the shift is small and your text is safely inside the spine, you're fine; if it's large, it's a page-count mismatch.