I have a RibbonWindow that I was previously using version 11.2.0554 to create, and somehow it had the glass effect even though I have my Windows settings set to "Windows Classic."
I recently upgraded to 12.1.0561, and now the default RibbonWindow does not restyle the maximize/minimize buttons. For reference, my window used to look like the picture (not the external sample) in the sample browser under Ribbon->RibbonWindow.