What is the KFX issue? I didn't keep up with all this stuff as all my books work well on my iOS devices. If the issue involves calibre, then I could not care less. I don't use it. I tried it years ago when eBooks, both ePubs and Kindle books, often needed format massaging, but I no longer feel the need to do so as I can comfortably read 99,999 books out of 100,000 on my iOS devices. Calibre was never an app I cared much for. It is way easier to just use the Kindle cloud rather than becoming a raging anal retentive librarian on steroids worrying I might not find a particular book within a millisecond of wanting it! If I get so old I can't remember where my books are, I'll be too old to read! I don't use my Macs or Windoze computers for reading books.
I have to giggle to myself when I read in this forum where some spend so much time modifying their books and organizing their libraries that they probably don't have time to actually read and enjoy their books.