Ah, you are on Mac, that changes things. There are only windows betas for now in that PR. My advice:
1) Try updating to Kindle Previewer 3.72 and moving kindlegen to /usr/bin/local. I feel like a slightly older version (3.71?) had some formatting issues that were fixed. Might be mis-remembering though.
2) Leave Stretch/Upscale unchecked.
3) I personally use MozJpeg and partially check split. Not sure if it will change anything but worth trying.
Maybe try running the latest master, instructions here:
https://github.com/ciromattia/kcc/wi...l-from-sources
I've had zero issues running KCC on either Windows or Mac, but I only run from source to get all the latest features and bug fixes.