Reproduction steps:
1. Open the attached “repro_IN-94108“ project
2. Open the “Assets/Scenes/SampleScene.unity“ Scene
3. Open the UI Builder (“Window → UI Toolkit → UI Builder“)
4. In the UI Builder Library, under “UI Documents (UXML) → Assets“, double-click “B.uxml“
5. In the UI Builder Inspector, in the “Attributes“ section, change the “Template“ parameter from ‘B' to any other (for example, 'C')
6. Save by pressing the “Ctrl + S“ button combination on your keyboard or by pressing “Save“ (“File → Save“)
7. Observe the UI Builder and the Console

Expected result: None of the elements disappear and the “Template“ parameter changes the value to the one that was set in the 5th step
Actual result: Elements disappear and the “Semantic - Unknown template name“ error appears in the Console

Reproducible with: 6000.0.37f1, 6000.1.0b5, 6000.2.0a2
Couldn’t test with: 2022.3.57f1 - “Attributes“ section doesn’t have the “Template“ parameter

Reproducible on: Windows 10 (22H2), macOS Ventura 15.2 (24C101) (user’s environment)
Not reproducible on: No other environments tested