My exported PDF no longer has the front and back covers. Why?
From version 1.9.20 the PDF no longer includes the covers by default. The PDF is the inside of your paperback: KDP prints it as the book's interior and wants it without the cover, which you upload separately as the cover wrap. To get a PDF with its covers, for example to read on screen or to send to someone, open the book's page, open the Export card, tick "Covers in the PDF" and export again. The setting is kept for that book. The EPUB and Kindle files keep their covers as before.