I don’t think this is something that comes with Scrivener; we have nothing called a “KDP Paperback” built for the software anyway, nor do I quite understand that would mean. Was it a compile Format you downloaded from somewhere? That’s fine if so, don’t get me wrong, it just helps to know what the context is.
What we do have is a “Paperback” compile format, in two sizes, one 6x9in and the other 5.06x7.81in. Paper size aside, the two are the same, and if you double-click on one of them to duplicate and edit, navigate to the Section Layout pane, and examine the text settings in any of the layouts that print text, you’ll find we use 10pt Palatino.
So that’s where I’m a bit confused, as you indicate it should be 12pt, and that this is what you compiled with.