For all except for the older e-ink Kindle devices, the Kindle Previewer has displayed only about 3/4 of the cover... with about 1/4 of the cover... at the far right side of the page, missing entirely. With Amazon Kindle Previewer, Version: 2.751, that appears to be fixed.
See screen-shots below. The first is a test file I created for this purpose, "Cathedrals Cover Test 1563 x 2500" (photo thanks to PunchyGonzales of the KDP Forum), as displayed on a physical iPad, testing thanks to Hitch of Booknook.biz.)
The second screen-shot is the same file, displayed in the new version of the downloadable Kindle for iPad Previewer.
|Cathedrals 1563 x 2500 pixels. Being tested on physical device, iPad by Hitch of Booknook.biz|
|Screen Shot, Cathedrals 1563 x 2500 displayed in Kindle for iPad Previewer.'|