Steps to repro:

1. create a uxml document with a bunch of controls ( or use the one attached file )
2. create a ui document in the unity hierarchy ( Create -> UI Toolkit -> UI Document )
3. Set the document to use the uxml document you created
4. Add a secondary game view ( ellipsis next to inspector -> add tab -> game )
5. You should now see your UI on both screen
6. Hit the resolution drop down ( free aspect ) going back and forth between both game view

Result:

You'll see that there's a rescale issue happening and even the background is sometimes not showing up.

!multi-display.gif!