Steps to reproduce:

Create a new Unity project using the Universal 3D template
Right-click in the Hierarchy > UI Toolkit > Standard Elements > UI Library
Double click the "Multi Column List View" or "Multi Column Tree View" to add it and save the .uxml in project
In the Inspector, observe that Stretch Mode is "Grow And Fill" and that Reorderable and Resizable are enabled
Expand the Columns > Columns foldout and add one Column using the "+" button
Expand the new Column and observe that Visible, Sortable, Optional and Resizable are enabled and Min Width Value is 35
Save the .uxml
Observe the same properties in the Inspector

Actual results: Unset attributes revert to type default values after saving - become unchecked or go back to "0" value

Expected results: Unset attributes keep their declared default values after saving

Reproducible with versions: 6000.6.0b2, 6000.6.0b7, 6000.7.0a4

Can’t test with versions: 6000.3.21f1, 6000.5.7f1 (in-scene UI Authoring not available)

Tested on (OS): Windows 11

Notes:

Reproduces only in in-scene UI Authoring. Editing and saving the same document in the UI Builder window is not affected - the default values persist correctly there
Manually enabling a property writes it explicitly to the .uxml, after which it persists correctly