From GitHub: https://github.com/Unity-Technologies/InputSystem/issues/383

Note: this happens when the Input Action's Type is set to "Any", "Axis" or "Vector2", a.k.a the ones that can take mouse scroll input as Binding path. It does NOT happen to the other Types.

Repro:
1. Download package and project at: https://github.com/Unity-Technologies/InputSystem
2. Open SimpleControls Input Action at: Assets -> Demo -> SimpleDemo
3. In the middle section, create a new Input Action and select it
4. In the right section, set the Type to Any, Axis or Vector2
5. In the middle section, select the Empty Binding under the new action
6. In the right section, click the "O" button next to the drop down menu for Path
7. Observe the Binding change
8. If nothing happens, cancel the binding. Scroll the mouse a little, then repeat step 6.

Actual: As soon as the user clicks the "O" button, the Binding is set to mouse scroll.