IF there's one of those books still in my amazon library, I will test, but from memory those books are offered ONLY in KFX and there is no way to get them onto a PC. even though current versions of K for PC can handle KFX. its another enforced limitation, on top of the single format only restriction. probably because a fire tablet can handle a "hardened "form of their DRM ?
they applied this limitation first to some graphic novels, then to some expensive academic tomes, and now , apparently, to some cookbooks also. there was a thread here somewhere about the academic books.
a similar system applies to prime lending - no matter how many fire tablets and serial numbers I have, I cannot get a prime lending book sent to a PC - not to K for PC, not to USb transfer...
NB that was only with prime lending, not Kindle Unlimited, and they may have renamed it again now
its been a while since I used it