Back to scratching mud
Aaron Shepherd is convinced that this is true. The .kfx (which Aaron believes stands for .kf10) is a post-uploading process done to your books over the first month or two after uploading. What it means as far as he is concerned is this:
In practical terms, this means there is no longer any effective way to proof your book before or after publication. You can get it looking absolutely perfect in a KDP proof, and even right after it goes on sale, only to have it skewered a few weeks later. You are asked to blindly entrust the formatting of your books to a team that has shown near-zero sensitivity and competence in typographic matters.
The typographic horrors imposed on your books (if this is all true) now make Amazon heir apparent to all the nasties of earlier Word formatting. Evidently they change paragraph spacing and a lot more.
Amazon needs to tell us what they want
This mud-scratching book production really needs to stop. They need to give us tools to use we can trust. A new InDesign export plug-in would be a good start.