Dragged selector is not persisted and gets removed from the Visual Element after saving when the selector was created in an external editor

Steps to reproduce:

1. Create a new URP project
2. In the Hierarchy, right-click and select UI Toolkit > Standard Elements > Visual Element
3. Observe UI Viewport and Style Sheets windows open
4. In the Style Sheets window, create a new selector (with a new StyleSheet)
5. Save (Cmd+S)
6. Open the created .uss file in external text editor
7. Create a selector with any name
8. Save, exit and observe the Selector appears in Style Sheets window
9. Drag the selector onto the Visual Element
10. Observe the selector is shown next to it and Save (Cmd+S)
11. Observe the Visual Element

Actual results: The selector is removed from the Visual Element on save

Expected results: The selector remains applied to the Visual Element after saving

Reproducible with versions: 6000.6.0b1, 6000.7.0a1

Can’t test with versions: 6000.4.12f1, 6000.5.1f1 (uxml editing In Context not available)

Tested on (OS): macOS 26.5