Create a TestPlugin that has platforms/android folder.
Execute:
tns create TestApp && cd TestApp
tns plugin add ../TestPlugin
tns run android
ls platforms/android/src/main/assets/app/tns_modules/test-plugin/platforms/android # no such dir
tns run android --release
ls platforms/android/src/main/assets/app/tns_modules/test-plugin/platforms/android # lists files
The second prepare doesn't delete the platforms folder of plugins.
Using NativeScript CLI 2.4 on macOS with Node 4.6.