It is not the 'map/ but the split between the view pane and text pane. In your scenario the view pane holds a treemap and the text pane is empty but normally holds info on the selected note.
In any document window tab, the two views are divided by a moveable splitter bar. In your first screen grab you have likely used the Window ▸ View only menu option or Cmd+6 to move the bar all the way right, essentially hiding the the text pane and giving over more space to the view pane.
If the tab (either or both panes) appears to need updating, i.e. the UI being redrawn correctly, clicking the current tab does this. In the past the same was achieved by switching to another tab and back.
For @eastgate, in testing, I noticed an additional separate treemap issue. The treemap view is also failing to correctly store/restore its selected item, i.e. tab switch can result in an empty view pane even though a note was selected when the tab was last used. Tab click does restore the view pane and splitter state but not the view’s selection. Looking that the XML, the selection parameter is not getting populated properly: