What controls where Kindle places the author in author sort?

Hello, I am posting this in the Calibre forum because it is in the context of using Calibre.

I have two .mobi files from Project Gutenberg of Gulliver's Travels, one I got some years ago and an updated one I got today. When I use Calibre to send the older one to my Kindle, it appears under "S" (for Swift). When I use Calibre to send the newer one to my Kindle, it appears under "J" (Jonathan, between books whose authors' last names begin with H and K).

I have edited the metadata so that, for both books, the Author field is "Swift, Jonathan" and the Author sort field is "Swift, Jonathan"; they are, as far as I can see, identical for the two books. I have looked in the metadata.calibre file on my Kindle and I see no relevant difference between the two books (other than a slight difference in title).

What is causing them to appear in different places in the sort on the Kindle? What can I change so that the updated version of this book appears under "S"? (Where is the emoji for "This is driving me crazy"?) :blink:

I would be happy to follow up with more details or the files themselves if it would help.

Thank you!