I have been wrestling for a good number of days with the validation problem:
Your APK is not 64-bit compliant. This may lead to some devices not being supported.
In my case, I have a Microsoft Xamarin application with a hybrid package built for both "armeabi-v7a" and "arm64-v8a". This seems to work fine with Android and Google Play, just failing this validation for Amazon Fire. It looks like others have had this same issue for example with Unity and that the validation is pretty picky.
When I do an APK build that is 32-bit only (armeabi-v7a) there is no validation error and it shows all the Fire Tablets I would like to support.
However, the support docs state "Some devices no longer support 32-bit apps...". Does this include any Fire Tablets?
If this is just other non-Amazon devices, I am inclined to just stay 32-bit only on the Amazon App Store.
