Android: excludeFromRecents not working

I added android:excludeFromRecents="true"

to AndroidManifest.xml.

But if I just stay in my app and click the Recent App button, my app is still showing up in the last app list.

If I press the Back button on the Launcher and press the Recent App button, my app won't show up in recent apps.

Is this an Android bug, or do I need to add some other flags to AndroidManifest.xml? Thank.