Steps to reproduce:

Open the attached project
Open the SampleScene
Select the PanelRenderer object in the Hierarchy and create a new UXML asset for it through the Inspector
Open the created asset in UI Builder and add a Label element
Save and close the UI Builder window
Expand the PanelRenderer object and click on the > button in the Scene Hierarchy to open the UXML asset in context
In the UI Viewport window, make any changes to the UXML file
Observe that the file is marked dirty as expected (Trying to exit the UI Viewport window with the < button should throw an unsaved changes pop-up)
Right-click on any UI element inside the UI Viewport window
Select the Open in UI Builder option
Observe that the UI Builder window is opened without the unsaved changes applied. The pop-up window for unsaved changes was not shown either
Check the UI Viewport window again

Actual results: Viewport window is no longer marked as dirty, and no unsaved changes are detected. The option to manually save the file is also disabled

Expected results: Selecting the option to Open UI Builder window should throw the unsaved changes pop-up, or the changes should carry over, marking both windows as dirty

Reproducible with versions: 6000.6.0b1, 6000.7.0a3

Can’t test with versions: 6000.0.79f1, 6000.3.20f1, 6000.5.4f1, 6000.6.0a7 (UI Viewport window not available)

Tested on (OS): macOS 26.5.2