How to reproduce:
1. Open the “IN-100074_TestUIBuilderNameSpace” project
2. Open Window > UI Toolkit > UI Builder
3. In the Library pane select “Project”
4. Observe available custom elements

Expected result: Custom elements that are defined in a namespace that starts with “Unity” are visible
Actual result: Custom elements that are defined in a namespace that starts with “Unity” are not visible

Reproducible with: 2023.2.0a9, 6000.0.48f1, 6000.1.2f1, 6000.2.0a10
Could not test with: 2022.3.61f1, 2023.2.0a8 ([UxmlElement] attribute is not introduced)

Reproducible on: macOS 15.4.1 (M1 Max)
Not reproducible on: no other environments tested

Notes:
- For reproduction the namespace must start with “Unity”, adding letters in front or using lowercase “u” will not reproduce the behaviour.
- Before version 2023.2.0a9 the “UxmlFactory” pattern is used for creating custom UI Toolkit elements. The behaviour does not reproduce with these attributes.