Steps to reproduce:
1. Open the attached “UnityCoreTesting.zip” project
2. In “Assets/Art” open “TestAnimation.fbx” in the Inspector Window
3. In the Inspector Window open the “Animation” tab
4. In the “Clips” section remove the “Armature|Idle_Bugged” clip
5. Add a new Clip
6. Select the New Clip
7. Open the Mask Foldout section
8. Observe that the Definition is set to “None”
9. Open the Transform Foldout section
10. Observe the results

Expected result: “No transform mask defined, everything will be imported” is shown
Actual result: The mask is applied from the “Armature|Idle_Masked” clip

Reproducible in: 2022.3.59f1, 6000.0.41f1, 6000.1.0b9, 6000.2.0a6

Reproducible on: Windows 10, Windows 11
Not reproducible on: No other environment tested