*Steps to repro:*
# create a uxml document with a bunch of controls ( or use the one attached file )
# create a ui document in the unity hierarchy ( Create -> UI Toolkit -> UI Document )
# Set the document to use the uxml document you created
# Add a secondary game view ( ellipsis next to inspector -> add tab -> game )
# You should now see your UI on both screen
# 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!