Steps to reproduce:

Open the attached "ReproProject" project
Open Window > QA > UUM149395 Unpack Redo Repro and click "Set up repro" (opens "Assets/UUM149395Repro/Root.uxml" in the UI Builder window)
In the UI Builder Hierarchy, right-click the "#ChildInst" element and select "Unpack Instance"
Undo (Edit > Undo or Ctrl/Cmd+Z)
Redo (Edit > Redo or Ctrl/Cmd+Shift+Z)
Observe the "#CA" and "#CL" elements in the UI Builder Viewport

Actual results: The "#CA" element loses its yellow background and 40px height, and the "#CL" Label loses its green text color.

Expected results: The "#CA" and "#CL" elements keep their inline styles after Redo

Reproducible with versions: 6000.6.5f1, 6000.7.0b3, 7000.0.0a5

Not reproducible with versions: 6000.0.84f1, 6000.3.26f1, 6000.6.4f1

Tested on (OS): MacOS Silicon Tahoe 26.6.2

Notes:

Also reproducible with "Unpack Instance Completely" on "#ChildInstForComplete". The nested "#GL" Label also loses its magenta color
Saving the document after Redo (File > Save) writes the elements to "Root.uxml" without their style attributes
After Redo, selecting "#CA" and changing Background > Color in the Inspector throws "NullReferenceException: Object reference not set to an instance of an object" (Unity.UI.Builder.StyleRuleExtensions.AddProperty), and the color is not applied
Duplicating an inline-styled element ("#RA"), then undoing and redoing, keeps its styles