Reproduction steps:
1. Open the attached “repro_IN-93239“ project
2. Open the “Assets/Test.unity“ Scene
3. Open the UI Builder (“Window → UI Toolkit → UI Builder“)
4. In the UI Builder Hierarchy, select the “ToggleButtonGroup“ element
5. In the UI Builder Inspector, scroll to the “StyleSheet“ section
6. Press “x“ on the “.hide“ selector so it’s removed
7. Save with the “Ctrl + S“ button combination on your keyboard or by pressing the “Save“ button (“File → Save“)
8. Observe the selectors in the “StyleSheet“ section

Expected result: The deleted “.hide“ selector is gone
Actual result: The deleted “.hide“ selector comes back to the selector list

Reproducible with: 6000.0.36f1, 6000.1.0b4, 6000.2.0a1
Couldn’t test with: 2022.3.57f1 - “ToggleButtonGroup“ not available yet in the UI Builder Library

Reproducible on: Windows 10 (22H2), Windows 11 (23H2) (user’s environment)
Not reproducible on: No other environments tested