Possible bug in Amazon Kindle with Marshmallow using SD for storage ?

[AC Question]

Have a k1 shield now running marshmallow with 128 gb sd configured as "internal memory" . Attempted to put kindle on sd and the books wouldnt download and the app kept disappearing. Books would que but never download . All other apps and data smooth as silk

 

[Peter Kwok]

The Kindle App sometimes disappears after reboot. I installed it again, not knowing that Amazon thought I added new devices. After several times, I could not download my books due to multiple devices. I have to sign onto Amazon from a PC to deregister my extra devices.
I moved Kindle back to internal memory.
Firefox, Flipboard, and few of my other apps also have problem of disappearing after reboot. They do not disappear at the same time.
BTW, I have a 2015 Moto X