Stage manager not working

The latest iPad Pro 12.9 running 16.1 Stage Manager was working great with my Dell monitor using a DVI to USBC cable. The monitor showed a separate desktop and I could move the apps and use Stage Manager as promised. Then I took my iPad to BestBuy to see if the Studio Display would use the display camera to unlock the iPad and when I plugged it in stage manager was reduced to only mirroring the iPad’s display. Stage manager was on. Apps were minimized and tiled. The display was mirrored and nothing would change it. I bring the iPad home and plug it back into my Dell monitor and the same mirroring issue.

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