When starting up the "marble" executable and looking at the legend all checkboxes are unchecked. This is apparently a bug since the features associated to the checkboxes are enabled.

Switching the map themes e.g. from Atlas to OpenStreetMap and back to Atlas will properly show the checkboxes checked. So there is a bug with the checkbox-state synchronization during Marble's start.

Using the "marble-qt" executable the bug is not reproducable. So the bug seems to be in the code related to Marble's KDE flavor.