That’s quite confusing as it is possible to write an annotation for a group that’s selected in the main view. Thanks!
For other readers, hidden preference, from help:
DisplayGroupsInPreviewPane: DEVONthink will display the contents of selected groups, smart groups, and feeds in the view/edit pane of main windows. The preview only displays files, not groups.
x-devonthink://preference?key=DisplayGroupsInPreviewPane&value=yes