Reproduction steps:
1. Open the attached “BugRepro” project
2. Go to the Assets folder
3. Righ-click and select “Update UXML Schema”
4. Observe the Console window

Expected result: No errors are thrown
Actual result: “XmlSchemaException: The Enumeration constraint failed.” error is thrown

Reproducible with: 1.1.0, 2.0.0-pre.11 (2023.3.0a2, 6000.0.27f1, 6000.1.0a4)
Not reproducible with: 1.1.0, 2.0.0-pre.11 (2021.3.46f1, 2022.3.52f1, 2023.2.0a23)

Reproducible on: Windows 11
Not reproducible on: No other environment tested

Note: The regression is in the Editor versions and not the package itself