Long time no see. Hope all is well.
I recently ran into this same issue, but not only in AppCenter, it was also happening in Android Studio. However, I was able to address it in Android Studio by following the steps in the top answer of this SO post:
After I followed the directions, my AndroidManifest.xml needed to be updated, specifically the application android:name value. But after updating that value and rebuilding the project I was able to get a stable build with a new package name.
Hope this helps and best of luck!