How to reproduce:
1. Create and open a new project
2. Create a new UI > UI Document GameObject in the Hierarchy and a UXML file in the Project
3. Open the UXML file in UI Builder
4. Add any visual element in the UI Document and save the changes
5. In the Hierarchy, select the UI Document GameObject and add the UXML file to the “Source Asset” field in the Inspector
6. Save the changes in the Scene, and build and run it
7. Observe the Player

Alternatively:
1. Open the “test_sample.zip“ project
2. Build and run the project
3. Observe the Player

Expected result: UI Document is rendered with all elements
Actual result: UI Document is not rendered at all

Reproducible in: 6000.1.0b13, 6000.2.0a8
Not reproducible in: 2021.3.50f1, 2022.3.60f1, 6000.0.45f1, 6000.1.0b12(8e2fce1fdf6e), 6000.2.0a7

Reproduced on: Windows 11 Pro (24H2), macOS 15.3.2
Not reproduced on: No other environment tested