I just updated Android Studio from 3.5.x to 3.6.x. After the update I had to delete the .idea/* and .iml files in order for the build to work due to AS complaining about mixing Gradle and Non-Gradle files.
Since then, when I press "Run app", the Gradle build completes successfully and an apk is actually generated in the build/outputs/apk folder but the APK is not automatically installed on the running emulator. I have to drag and drop the .apk myself. Furthermore, when I manually run Build->Build APKs I get the message Build APK(s): APK(s) generated successfully for 0 modules:
Both the instant run and the "don't install APK if unchanged" settings have been disabled. I am using Gradle version 4.3 and the Android Gradle Plugin version 2.3.3
Update: More recent versions than 3.6.x run fine again without any fixes on my part needed. So I guess thanks, Google/Jetbrains.