How to reproduce:
1. Create a new Unity project
2. In the Project window, right-click -> Create -> UI Toolkit -> UI Document
3. Open the UI Document and navigate to the StyleSheets tab
4. Create a StyleSheet, save it and then remove it
5. Click the "+" button -> Add Existing USS -> previously created StyleSheet
6. Observe the results

Expected results: The added .uss file is not dirty
Actual results: The added .uss file is dirty

Reproducible with: 2022.3.60f1, 6000.0.43f1, 6000.1.0b11, 6000.2.0a7

Reproducible on: macOS Sequoia 15.3 (M1)
Not reproducible on: No other environment tested

Note: In 2022.3 stream there seems to be no indication of unsaved changes on the .uss header but it seems to be reproducible there