How to reproduce:
1. Open the attached project "Repro_UITAuthoring"
2. Drop the "Assets/UITAuthoringRepro/NestedOverride.uxml" into the scene and click "Edit in UI Viewport"
3. Observe that the "lab" Label reads "OVERRIDDEN"
4. Right-click the "outer" template instance in the Hierarchy and select "Unpack Template Completely"
5. Observe the results
Expected results: The unpacked elements keep the overridden value and the Label still reads "OVERRIDDEN"
Actual results: The override is silently discarded and the Label reverts to the template default "leaf default"
Reproducible with: 6000.7.0a6
Can't test with: 6000.3.17f1, 6000.5.10f1, 6000.6.0f1, 6000.7.0a5 (No such functionality)
Reproducible on: Ubuntu 24.04.4 LTS (x86_64), macOS Tahoe 26 (M1)
Not reproducible on: No other environment tested