I usually prefer to use 'ragged right' text alignment, but decided to try 'justified' alignment.
As it happens, the very first book I tried this out with, and which supports 'advanced typography', fails to justify. There's also no hyphenation. Same behavior with the iOS app and on other Kindles.
https://www.amazon.com/gp/product/B08WLWC6GZ/
I tried several other books but they justify just fine.
I converted AZW3 to KFX with calibre, side-loaded, and same result: no justification.
Curious to know if others have seen this and have figured out what might be going on.
Otherwise I guess I'll go back to my 'ragged right' life.