New UXML Template GameObject is created instead of adding Standard Element to the existing UXML Template GameObject set as Default Parent

Reproduction steps:
1. Create a new project
2. Create a Standard Element (e.g. GameObject > UI Toolkit > Standard Elements > Button) and save UXML file
3. Exit the UI Staging Mode
4. Right-click the UXML Template GameObject in the Hierarchy and select "Set as Default Parent"
5. Create another Standard Element (e.g. GameObject > UI Toolkit > Standard Elements > Label)
6. Observe the Hierarchy

Actual result: New UXML Template GameObject is created in the Hierarchy and the Element is added to it
Expected result: Element is added to the UXML Template GameObject that is set as Default Parent

Reproducible with: 6000.7.0a6
Not reproducible with: 6000.6.1f1, 6000.7.0a5
Couldn’t reproduce with: 6000.0.79f1, 6000.3.19f1, 6000.5.3f1 (No Standard Elements option)

Reproduced on: Ubuntu 24.04, macOS Tahoe 26.6 (M4)