It seems that Amazon has somewhat modified its EPUB conversion pipeline to be somewhat more resilient some problems.
I got a bug report in my tools (
https://kindle-epub-fix.netlify.app/) that said that their book has empty table-of-content when using Send-to-Kindle.
Upon investigation, I find that the file in question has out-of-order TOC. The file failed entirely to convert using kindlegen, but it seems to be accepted by Send to Kindle. I have not independently verify this since I don't have a Kindle I am willing to put online at the moment.