If you open the build.gradle in Android Studio and debug, Android Studio will automatically add testOnly=true which makes it difficult to iterate with the editor since -t is needed when installing again. It can also cause some devices (like OPPO) to fail to install.
The APK will have testOnly set to true so the APK (verify by looking in Intermeidate/Android/APK/gradle/app/build/intermediates/merged-manifests/debug/AndroidManifest.xml. We do NOT want this.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80074 in the post.