Update.
I received two separate email replies from Amazon Tax Central. One was an Excel listing of all payments to me in 2018. A curious feature of this is that Amazon for the first time ever included payments actually made on on January 2, 2019 as 2018 payments, and Amazon did not include the payments made on January 2, 2018. In prior years, my numbers from Amazon matched the actual dates I received them at my bank. This isn't Amazon using the accrual accounting system. It's still the cash accounting system but they're using their posting date, not the it's-really-in-the-bank date. This is analogous to receiving a physical check on January 2 that is dated December 29. In my case the dollar amounts were minimal, but it could mean a large discrepancy for authors who routinely sell a lot of books worldwide, since those payments were from international sales.
The Excel spreadsheet did have the correct payments this time. However, the dollar amounts did not add up to what is listed on the 1099-MISC, which remains unchanged in the Amazon Tax Portal. I've requested that Amazon change the 1099-MISC to reflect the true total on the Excel spreadsheet Amazon sent me.