Steps to reproduce:

1. Create and open an empty project and install com.unity.behavior
2. Create and open a behavior graph (Assets > Create > Behavior > Behavior Graph)
3. Add a "GameObject" blackboard variable
4. In the scene hierarchy, create a new game object
5. Add a behavior agent component
6. Assign the graph
7. Make two other prefabs (prefab1 and prefab2) and assign prefab1 to the variable on the scene game object agent (make sure you select the "Assets" tab in the object picker)
8. Drag and drop the game object to the project view to make a prefab out of it
9. Assign prefab2 to the variable on the scene game object agent
10. Apply the changes to the prefab by right-clicking on the component (see video)
11. Click on the prefab in the project view and observe the value of the blackboard variable

Actual results: The blackboard variable on the agent prefab references prefab1

Expected results: The blackboard variable on the agent prefab references prefab2

Reproducible with versions: 1.0.8

Tested on (OS): MacOS