Reproduction steps:
1. Download and open the attached project called "butterflies-e2.zip"
2. Go to Assets/Anim/anne in the Project window
3. Go to Animator window -> "Base Layer" -> "ground" -> "ground" -> "Brake" Blend Tree and observe the referenced Animation Clips
4. Got to Assets/Graphics/Models/Anim/mikeRider_movement in the Project window
5. In the Inspector window, add a new Animation Clip to the list
6. Choose "Armature|Body.Wallride.001" as the Source Take
7. Check Loop Time and Loop Pose, then click on Apply
8. Reimport the "mikeRider_movement" model
9. Observe the previously mentioned Blend Tree and all the others around it

Expected result: References to Animation Clips do not disappear
Actual result: All references to Animation Clips, which came from the reimported model, disappear

Reproducible with: 2019.4.10f1, 2020.1.6f1, 2020.2.0b3
Could not test with: 2018.4.27f1 (due to errors in Console when opening the project)

Notes:
- Reproduced on Linux, macOS, and Windows
- A reproduction video called "2020-07-31 13-01-37.mkv" is attached below