Saving moves Toggle Button Group buttons to the wrong place in hierarchy

Steps to reproduce:

1. Create a new URP project.
2. Assets > Create > UI Toolkit > UI Document and open it in UI Builder.
3. From the Library, add a ToggleButtonGroup to the hierarchy.
4. From the Library add three Buttons as children of the ToggleButtonGroup.

Either: 

• Save

Or:

• Undo and redo.

Actual results: All child buttons exit the ToggleButtonGroup, and remain outside.

Expected results: Buttons should remain children of the ToggleButtonGroup.

Reproducible with versions: 6000.1.0a4, 6000.0.28f1, 6000.0.20f1

Not reproducible with versions: 6000.0.19f1

Can’t test with versions: 2022.3.52f1, could not find ToggleButtonGroup

Tested on (OS): Ubuntu 24.04