How to reproduce it:
1. Open the attached project "1319903.zip"
2. In the Project window right-click on 'PARENT.uxml' and click on 'Open C# Project' context menu item
3. Notice that the 'PARENT.uxml' file contains 'CHILD.uxml' reference and close it
4. In the Project window rename the 'CHILD.uxml' file to 'CHILD_RENAMED.uxml'
5. Open and observe the 'PARENT.uxml' file
6. Observe the Editor's Console window

Expected result: In the 'PARENT.uxml' file 'CHILD.uxml' reference is changed to 'CHILD_RENAMED.uxml', and there are no errors in the Console window
Actual result: In the 'PARENT.uxml' file 'CHILD.uxml' reference is not changed, and errors appear in the Console window

Reproducible with: 1.0.0-preview.14 (2020.3.0f1, 2021.1.0b11)
Could not test with: 2018.4.32f1, 2019.4.22f1 (UI Toolkit is not introduced yet), 2021.2.0a8 (due to UI Toolkit's errors)

Errors that appear in Actual result:
- Assets/PARENT.uxml (1,2): Semantic - The specified URI does not exist in the current project : Assets/CHILD.uxml
- Assets/PARENT.uxml (4,10): Semantic - Unknown template name 'CHILD'