For about a week now, the Look Inside sample on the Kindle store has come in two flavors, and the flavors can vary over time. At one point, most of my books were New Style, but today it seems that only about half of them are. The treatment does not vary by upload date. For a while I thought perhaps it was unique to books uploaded in epub format, but that no longer makes sense because I upload all my books that way.
To see the two styles (at least at this moment), I opened the top two Kindle best-sellers. The first is The Things We Wish Were True by Marybeth Mayhew Whalen Link:
https://amzn.com/B019K2JGYE
That's what I call Old Style. The Look Inside begins with the cover and scrolls through all the following content.
The number two best-seller is Everything We Keep by Kerry Lonsdale
https://amzn.com/B0184ZLSN0
That's New Style. When I click on such a book cover, the Look Inside does a little dance, flicks through several pages, and finally settles on the first page of chapter one. (And sometimes in the middle of earlier matter, such as a pair of epigraphs I saw on one sample, the first being ignored and the second -- by Joseph Goebells, no less -- display.) I think the font is different, too, though not as different as I see in my own books, where not only has the font changed but it doesn't seem to follow normal 1.2 line spacing.
Not only that, but there have been complaints on the KDP forum of books changing their format in recent downloads. One complained that the second half of his book was all italics. Now, that can happen in Look Inside, when there's an unclosed italics (or bold, or center) tag. But within the body of the book, an unclosed tag ought to be closed out automagically at the end of the paragraph that contains it.
FWIW!