If you export a book to jpeg and have the Page Grid turned on under guides. The guide lines actually export in the jpeg! Guide lines are not supposed to show in the export, and they don't show in a pdf export. I almost send in an order with grid lines on the pages.
Updates to Lightroom Classic CC (8.2), Lightroom CC for desktop (2.2), Lightroom CC.Mobile (Android and iOS) were released on Monday, February 11, 2019. Please verify that your issue is fixed by the current update.