This came up in https://musescore.org/es/node/297132.
When you use File->Open or the icon on the toolbar, it opens a window showing the files in the selected folder. At the top right of this list is an icon that says Show preview pane. Apparently I turned this off several years ago (like versin 2.0.3) because it didn't do anything for me. Oscar in the linked post says that in previous versions he would use the Score preview pane to see a preview of his score and it now no longer works on his Linux System.
Any info on this would be appreciated.