After accessing the SwiftKey keyboard (ie touching a TextField) and closing it (pressing enter or hiding it) the application can't receive Keys events anymore. UPDATE: This is only for the back button.
I created a simple application to test it, if you press a key, for example the back button, the application log the event. After accessing the SwiftKey keyboard if you press the back button various times you see that not all the Keys event are logged (sometimes it log one or two events then it stops).
The bug seems to be triggered only with the SwiftKey keyboard on Android, the standard keyboard works as expected.