Reproduction steps:
1. Open the attached project "FieldNoSave"
2. Open “Assets/SerializedPropertyIssue/NewUXMLTemplate.uxml“
3. Click on “Element” in UI Builder Hierarchy
4. Set M_value Path to “abc“
5. Save “NewUXMLTemplate.uxml“ (Ctrl + S)
6. Close UI Builder window
7. Repeat steps 2 and 3
8. Observe “Path“ value under “M_value“

Expected result: “abc“ is displayed
Actual result: Path value is empty

Reproducible with: 2023.2.0b13, 2023.3.0a8
Couldn’t test with: 2021.3.31f1, 2022.3.11f1, 2023.1.17f1 - “error CS0246: The type or namespace name 'UxmlElementAttribute' could not be found“

Reproducible on: Windows 10
Not reproducible on: No other environment tested