Steps to reproduce:

Create a new project from the Universal 3D (URP blank) template
Select GameObject > UI Toolkit > Standard Elements > Button
In the "Create UI Document" dialog, keep the default name "New UXML" and click Save
In the Hierarchy, expand New UXML and observe that the document is shown as "New UXML.uxml*" with the Button under it
Click an empty area of the Hierarchy so that nothing is selected
Enter Play Mode, then exit Play Mode
If the "UI Assets Have Been Modified" dialog appears, click Save
Observe New UXML in the Hierarchy
Select File > Save
Quit the Editor. If a save dialog appears, click Save
Reopen the project and expand New UXML in the Hierarchy

Actual results: After exiting Play Mode the "*" disappears and no save dialog is shown; File > Save and quitting do not save the Button, and it is gone after reopening

Expected results: The Button is kept: the document stays marked as modified, or the save dialog is shown on exiting Play Mode

Reproducible with versions: 6000.7.0b4, 6000.7.0b5, 7000.0.0a7, 7000.0.0a8

Not reproducible with versions: 6000.7.0b3, 7000.0.0a5

Can't test with versions: 6000.0.84f1, 6000.3.24f1, 6000.6.6f1 (in-scene UI authoring is not available)

Tested on (OS): macOS 26.6.2

Notes:

Not reproducible when the document was saved once before the element was added: the "*" survives Play Mode and File > Save writes it
If the Button is still selected during Play Mode, "InvalidOperationException: You can't modify a VisualElement after its resources are released" is logged and the Inspector goes blank (~~UUM-155224~~). The element is lost with or without the selection
Workaround: select File > Save while "New UXML.uxml*" is shown, before entering Play Mode