All controls on the widget snap back to the default.
When you open/edit or create a "List Category Posts" widget and select a category and click save, the category snaps back to the first category in the list. The plug-in remembers your choice, but it will not display that choice within the widget manager.
I have to assume this is due to a change in 3.1 and how the plug-in's widget polls the saved value of the configuration?
I've got Version 0.18.3 of List category posts installed