Repro:

1. Create an input action with a Vector2 composite.
2. Bind "Up" to "W", "Left" to "A", and "Right" to "D".
3. Do *NOT* bind "Down" to anything.
4. Add a PlayerInput to the scene and link it to the actions.
5. Set it's behavior to "Invoke Unity Events" and hook up the action from above to PlayerInput.DebugLogAction.
6. Go into play mode.
7. Hit "W" and observe that a message is logged to the console.
8. Hit "A" and "D" and observe that in both cases no message is logged, i.e. the inputs aren't functional.