I have a paid unity game app on the Amazon App Store and Amazon GameCircle works fine for that version.
I also have the same app on Amazon Underground which is signed with the same keystore as the paid Amazon App
I added it to the existing GameCircle configuration and Security profile - this time the dev console also asked for an SHA256 signature which I provided. I downloaded the new underground version api key and updated the api_key file in my project.
While testing the Underground version works fine with GameCircle. Once it becomes live at Amazon however it does not work in the app which users download from Amazon Underground. No GameCircle pop ups or anything at all. It works fine in the apk I uploaded to Amazon Underground though.
I also uploaded the apk to the App Testing Service - I can see GameCircle integration is working from the test screenshots there too.
I created a second security profile for the Underground app which allowed me to create an API key for GameCircle which only required an MD5 signature to see if this would work...again GameCircle worked in testing but after uploading to Amazon it fails to work for users who download from Amazon Underground.
Strange that it works fine in the Amazon App Store version but not Amazon Underground?
In this thread https://forums.developer.amazon.com/answers/38063/view.html a users solution was re-using the same api key in both app versions. I tried re-using the key in testing and it didn't work.
It seems to be breaking when Amazon add Underground on to the app?
 I see the following in logcat from the released Underground App:
10-18 01:55:32.738 14399-14399/? D/com.amazon.identity.auth.device.appid.APIKeyDecoder.PII: APIKey: 10-18 01:55:32.738 14399-14399/? I/com.amazon.identity.auth.device.appid.APIKeyDecoder: num sigs = 1 10-18 01:55:32.738 14399-14399/? D/com.amazon.identity.auth.device.appid.APIKeyDecoder.PII: Signature checking.: 10-18 01:55:32.738 14399-14399/? D/com.amazon.identity.auth.device.appid.APIKeyDecoder.PII: Fingerpirint checking: 10-18 01:55:32.738 14399-14399/? W/com.amazon.identity.auth.device.appid.APIKeyDecoder: Failed to decode: Decoding fails: certificate fingerprint can't be verified! java.lang.SecurityException: Decoding fails: certificate fingerprint can't be verified! at com.amazon.identity.auth.device.appid.APIKeyDecoder.verifyPayload(APIKeyDecoder.java:332) at com.amazon.identity.auth.device.appid.APIKeyDecoder.doDecode(APIKeyDecoder.java:127) at com.amazon.identity.auth.device.appid.APIKeyDecoder.decode(APIKeyDecoder.java:91) at com.amazon.identity.auth.device.appid.AbstractAppIdentifier.getAppInfo(AbstractAppIdentifier.java:85) at com.amazon.identity.auth.device.authorization.api.AmazonAuthorizationManager. (AmazonAuthorizationManager.java:123) at com.amazon.ags.api.AmazonGamesClient.initialize(AmazonGamesClient.java:144) at com.amazon.ags.api.unity.AmazonGamesClientProxyImpl$1.run(AmazonGamesClientProxyImpl.java:68) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:158) at android.app.ActivityThread.main(ActivityThread.java:7225) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 10-18 01:55:32.738 14399-14399/? E/GameCircleClient: Unable to read api key. GameCircle requires an api key to be included in 'assets/ api_key.txt'. GameCircle will not initialize.