New selector is added to the top-most stylesheet instead of the active USS when using "Add new selector" in UI Builder

How to reproduce:

1. Open the attached "IN-144305" project
2. In the Project window, open “Testing.uxml”
3. In the UI Builder's StyleSheets panel, right-click “SecondSheet.uss” and select “Set as Active USS”
4. In the Add new selector field, type any selector name and press Enter
5. Observe where the new selector is placed

Actual result: The new selector is always added to the top most stylesheet in hierarchy
Expected result: The new selector is added to the active style sheet

Reproducible with:6000.3.0a2, 6000.3.17f1, 6000.4.10f1, 6000.5.0b11, 6000.6.0a6
Not reproducible with: 6000.0.0f1, 6000.0.76f1, 6000.3.0a1

Reproducible on: macOS 26.5 (M1), Windows 11 (reported)
Not reproducible on: No other environments tested