Steps to reproduce:

Create a new 3D URP project
Create a new UI Document and open it in UI Builder (Assets > Create > UI Toolkit > UI Document)
In the StyleSheets pane, click “+” > “Create New USS” and save it
In the selector field, type “.test” and press Enter
Drag a Label from the Library into the Hierarchy
Drag the “.test” selector from the StyleSheets pane onto the Label in the Hierarchy
With the Label selected, in the Inspector set Size > Width to 50px (or change any other property value)
In the StyleSheets pane, select “.test”
In the Inspector, set Display > Opacity to any different value
In the Hierarchy, select the Label
In the Inspector, right-click Display > Opacity and choose “Open selector in IDE”
Observe the Console and UI Document

Actual results: The Label loses its 50px width, the UXML is no longer marked as having unsaved changes, and “Accessing invalid property” is logged in the Console

Expected results: The Label keeps its 50px width and the UXML stays marked as having unsaved changes

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

Not reproducible with versions: 6000.6.6f1, 6000.7.0b3

Tested on (OS): macOS 26.6

Notes:

No unsaved-changes prompt appears when closing UI Builder afterwards, because the document is no longer marked as unsaved