How to reproduce:
1. Open the attached "IN-140720_UIBuilderEventListOverrideIssue.zip" project
2. In the Project tab, double-click "Assets/Repro/Instance.uxml" to open it in UI Builder
3. Select the "target" element in the Hierarchy
4. In the Inspector, add a new item to the list-value attribute, so that the list has three items: a, b, c
5. Right click the TemplateContainer for Base.uxml and open it in isolation
6. In Base.uxml, select "target" element
7. In the inspector, reorder the list to (b, a) (swap the two items) and save (Ctrl+S)
8. Go back to Instance.uxml
9. Observe that the order in "List Value" is correct (b, a, c)
10. Close Instance.uxml in UI Builder, then reopen it from the Project tab
11. Select "target" in the Hierarchy
12. Observe the "List Value"
Actual result: Items in the "List Value" are in the wrong order - a, b, c
Expected result: Items in the "List Value" are in the same order as set in "TemplateContainer" opened in isolation - b, a, c
Reproducible with: 6000.3.14f1, 6000.4.4f1
Could not test with: 6000.0.74f1, 6000.5.0b6, 6000.6.0a4 (Errors thrown in Console related to missing namespaces caused by API changes)
Reproducible on: Windows 11
Not reproducible on: No other environments tested
Workaround: Reimporting the instance asset shows the correct order
Note: The bug only manifests when the project uses a custom “UXMLImporterImpl” subclass that rewrites instance attribute-override values at import time