Reproduction steps:
1. Open the attached “BugRepro” project
2. Open the “Assets/OverrideTest-Override.uxml” Asset in the Project window
3. Select the “TemplateContainer/Example” from the UI Builder Hierarchy
4. Override an attribute (Set “Focusable” to checked)
5. Save the Asset (File > Save)
6. Observe that the “Test Document” and “Test Material” attributes now appear as “None”
7. Unset the attribute that was overridden (Right click on “Focusable” and press “Unset”)
8. Observe that the “Test Document” and “Test Material” attributes now have their values back

Expected result: Overriding attributes does not affect other attributes
Actual result: Overriding attributes cause other attributes to be null

Reproducible with: 6000.0.22f1
Couldn’t test with: 2021.3.44f1, 2022.3.50f1 - No support for “UxmlElement” or “UxmlAttribute” attributes

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

Notes: Inspecting the uxml source while reproducing, it appears that the attributes “TestDocument” and “Test Material” are not actually overridden/set to null. Also, while Unset appears to restore the values of “Test Document” and “Test Material”, the Unset operation doesn't work (note that the UI Builder window doesn't show any changes '*'). If you reload the file, you can see that the attributes still appear to be null.