Hi @franksmcb,
I was able to repeatedly reproduce that on UM 20.10 in VirtualBox. This happens with apps using CSD, as was mentioned before. All you need is to force your app to start maximized, and then unmaximize it by pressing window button or grabbing headerbar and dragging. The easiest way to start the app maximized was to maximize app, close it while maximized and relaunch. The bug occurs with "Do not auto-maximize new windows" option both on and off.
Cheers