Several people have reported that some books marked as "Enhanced Typesetting: Enabled" do not show the enhanced typesetting features on their devices. I decided to investigate this using two devices that support enhanced typesetting, a PW2 running firmware version 5.6.5 and the Kindle for iOS app version 4.10 on an iPad. On both devices I tested books from my Amazon account that show "Enhanced Typesetting: Enabled".
Only about 30% of the books in my Amazon account support enhanced typesetting according to their descriptions.
Of these, only about half download as kfx to the kindle. Examples of books that do not are
On Basilisk Station and
Veiled. I have tried deleting and re-downloading multiple times and the results are consistent for each book.
And only 75% of those that download as kfx to the kindle download as kfx to the iOS app. Examples of books that do not are
Life on the Mississippi and
The Last Praetorian. Looking that the files for these books on my kindle I found that all of these books contain the string "reflow-section-size" in their metadata.kfx file, while none of the books that download as kfx to the iOS app contain that string.
All of the books that download as kfx to the iOS app also download as kfx to the kindle.
Following is my speculation based on these observations.
It appears that Amazon developed an entirely new rendering engine and book format to support enhanced typesetting. In
another thread it was reported that the old rendering engine gave poor performance when kerning was enabled. Perhaps the Lab126 engineers decided it was better to start over than to try to improve the quality and performance of that rendering engine.
Because this would be a huge undertaking they may have decided to implement it in phases. First, supporting the features most used by e-books and then later adding support for more exotic or difficult features.
It could be that the iOS app has an earlier generation of the rendering software, supporting books needing only the smallest subset of kfx features. The PW2 may have a newer rendering engine supporting additional features. The books that do not download as kfx for either may reflect the next unreleased version of the rending engine. Perhaps, after a few more iterations, the new rendering engine may be able to support most Kindle books.
ETA: As can be seen later in this thread some books I am unable to download in KFX format are specific to my account because I bought them previously. I will need to rethink my analysis of this.