[stumped]

How exactly are you sideloading?

I use the calibre companion app to get books from calibre to my Amazon tablets and in all cases it places the ooks in books ,not documents

I don't have the exact same device as you but they should all behave the same.

So if you have been doing it manually with a usb cable. Get the trail version of the app instead and see if works. If not there is a sub forum where you can ask for more help

Ps if all but one of your devices file the sideloads correctly that suggests that it is not a problem within the books. Or within calibre, it's device settings or it's where you are putting them.i.e.in what folder.

But there is no such thing as a Kindle fire oth gen. There are kindles which are sink readers and there are fires, which are android tablets but the Kindle part of the Kindle fire name was dropped years ago. So what is the device you have called on an Amazon listing and in device info, and what version of fire os is it using. And is it fully updated. Press check for updates