Well, kdp is reviewing the book.
When I finished it, I went back to see if every file was complete (45 files, ugh) and found one that had been truncated, but it was near the beginning of the book and I think may have happened before I took everybody's advice and stopped using book view. And keeping preview open and making sure there were no errors before doing anything.
Did a proofread and corrected a bunch of errors, saved and checked for completeness again, and said, enough, done.
Oh, on my proofread, I found an error that Sigil missed, <p clas="nib tb">, imagine that. I was proofreading on Kindle for PC and the para looked like all the others. When I opened the file in Sigil, preview did not indicate any error.
So, even tho I've proved to myself that it can be used without losing files, it still has me so paranoid that I feel compelled to check that every file is complete, and that is a pain.
And now back to the book I was working on when I quit Sigil. That one is a court case with almost 300 witnesses, I don't even know how many files. When I finish it, I'll probably rework my own program for general use. A program that deletes your work, for whatever reason, especially without any notice that it has done so, is flawed, imho.