@LastSilmaril: Your problems might be some aspect of known and longstanding bugs with fonts cascade / fallbacks configuration and some CSS combinations.
Although it was observed with KF8 books I wouldn't be suprised if same thing applies for KFX books.
Read my comment
here.
It's probably some of those "invalid" CSS scenarios that's at the root of the problem. You could try testing those problematic books like this:
manually download the books to your PC from Manage
Your Content and Devices (that way you will get azw3 file [= KF8 format] even for the books that Kindle itself downloads as KFX), transfer them to you Kindle via USB cable.
Check if choosing any other build-in fonts work for a particular book or is the book stuck on same font (probably Bookerly)... check what happens when you enable "Publisher font".
Then, for the "buggy" book you should deDRM it and open it in Calibre Editor and check the CSS stylesheet for those things I've mentioned in that other topic (mainly if there is declaration of font family for body tag selector, and if there are references for font families
that are not embedded).
It really can't be said that it's the fault of font hack that Kindle and Amazon are doing bunch of "smart" heuristics in attempt to make "converted from a Word document" books display text using Bookerly at all cost... The devices should just correctly interpret CSS code.. since there is no forcing ebook publishers to always use CSS that Amazon deems "the only correct way".