Steps to reproduce:

1. Create a game object with a behavior agent component
2. Create a behavior graph and assign it to the agent
3. Open the behavior graph
4. Add a "Log Variable" node
5. Connect the "Log Variable" node to "OnStart"
6. Set the variable to log to "Self"
7. Save
8. Undo
9. Save
10. Enter play mode

Actual results: "[Agent] Self = Agent (UnityEngine.GameObject)" is logged to the console

Expected results: "[Agent] <no name> = null" is logged to the console

Reproducible with versions: 
1.0.9

Not reproducible with versions: 
1.0.8

Tested on (OS): MacOS

Notes:

• The attached screen recording shows it the other way around, but it's the same issue
• You can open project settings and add {{BEHAVIOR_DEBUG_ASSET_IMPORT}} to Scripting Define Symbols (domain reload will happen) to verify if the graph rebuilds when saving (You should see "GraphAsset[{}Behavior Graph{}].BuildRuntimeGraph" in the log window)