Reproduction steps:
1. Open the “ReproProject“ project
2. Open the “Assets/AnimatorController“ Animator Controller
3. Select the “BlendTreeState“ State in the Animator window
4. Change the Motion property from “Blend Tree“ to “Assets/Animation“ Animation in the Inspector window
5. Save the project (File > Save)
6. Open the “Assets/AnimatorController“ Animator Controller with a text editor application
7. Search for “BlendTree“
8. Observe the search results in the text editor application
9. Create a new Blend Tree in State (Right Click the “BlendTreeState“ State > Create new Blend Tree in State) in the Animator window
10. Repeat steps 5, 6, 7, 8
Expected result: Only one Blend Tree reference exists
Actual result: The old Blend Tree reference exists as well as the newly created Blend Tree reference
Reproducible with: 2021.3.35f1, 2022.3.19f1, 2023.2.9f1, 2023.3.0b6
Reproducible on: Windows 11 Pro (22H2)
Not reproducible on: No other environment tested