Cannot change certain Visual Element properties when the Visual Element was saved and changing properties in the Inspector window

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Create a new Visual Element in the Hierarchy (UI Toolkit > Standard Elements > Visual Element)
3. Click the "Continue" button to open UI stage
4. Save the Visual Element (File > Save/Command + S)
5. Add a Variable in the Variables list
6. Add Style Class to List and then remove it by pressing the "x" button
7. Observe the Inspector window

Actual results: Variables are not created and cannot remove elements in the Style Class List

Expected results: Variables are created and elements can be removed from the Style Class List

Reproducible with versions: 6000.6.0a7

Can’t test with versions: 6000.0.0f1, 6000.0.77f1, 6000.3.17f1, 6000.4.11f1, 6000.5.0f1 (feature not available)

Tested on (OS): MacOS Silicon Tahoe 26.3, Ubuntu 24.04