I have been experimenting with Kindle for Android under BlueStacks with Nougat 64-bit and have seen some odd behavior. I used Kindle for Android versions 4.16.0.75 and 8.51.1.0.
The 8.51 version was released late last year and is the newest version that is supported on older Android devices. I own a Nexus 7 tablet running Android M and that is the most recent version available from the Google Play Store for that device. Newer Android devices are currently at version 8.73.
Kindle for Android 8.51 on my Nexus 7 downloads most books in KFX format. However the same APK installed under BlueStacks N64 downloads the same books in KF8 format with a .prc extension instead. There are tricks that can be used to force Kindle for PC/Mac download KF8 instead of KFX. It seems that running under BlueStacks has the same effect on Kindle for Android!
There is a significant difference between K4A 4.16 and 8.51 in the book files downloaded. Version 4.16 downloads KF8 files with standard definition images, equivalent to what you get using Download & Transfer from amazon.com. However version 8.51 downloads an additional AZW6 file for many books with higher quality images.
Higher resolution images from Kindle for PC can be processed when adding to calibre using the
AZW6 Image Merge plugin. And I previously created
an update to that plugin to handle files from e-ink Kindles. I may produce another update to also handle high definition images from Kindle for Android if there is any interest.