Reproduction steps:
1. Open the attached “BugRepro” project
2. In the Project window, select “Assets/TestTheme” Theme Style Sheet
3. In the Inspector window, observe “Style Sheet 1” and “Style Sheet 2” Style Sheets in the list
4. In the Project window, rename the “Assets/Styles” folder to anything else (e.g. “Styless”)
5. Repeat step 2 and 3

Expected result: The two mentioned Style Sheets are still in the list
Actual result: There are two missing references in the list

Reproducible with: 2021.3.43f1, 2022.3.45f1, 6000.0.18f1

Reproducible on: Windows 11, macOS 13.2.1 (M2 Pro)
Not reproducible on: No other environments tested