How to reproduce:
1. Open the “UITK Testing U6.zip“ project
2. Open the “CreateMenuExample.uxml” file in UI Builder
3. Observe the UXML Preview section at the bottom of the UI Builder
4. Open the “UIBuilderExample.uxml“ file in UI Builder
5. Observe the UXML Preview section at the bottom of the UI Builder

Expected results: All elements in both UXML files feature the same or no prefix
Actual results: Elements in the “CreateMenuExample.uxml” file feature the “engine:“ prefix and elements in the “UIBuilderExample.uxml“ feature “ui:“ prefix

Reproducible in: 2023.3.0b5, 6000.0.42f1, 6000.1.0b10, 6000.2.0a6
Not reproducible in: 2022.3.60f1, 2023.3.0b4

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

Note: Downgrading and upgrading the project between reproducible and not reproducible versions does not break the prefixes (after reupgrading, it still shows differently), so, in the not reproducible versions, the prefix information likely still gets serialized differently based on the way it was created