As far as I know the document tree has always displayed the document title.
However, if you really, really want it to display the menutitle instead, you can make one change to the /manager/frames/nodes.php file. On line 98 change the query being built to have menutitle instead of pagetitle:
$sql = "SELECT DISTINCT sc.id, menutitle, parent, isfolder, published, deleted, type, menuindex, hidemenu, alias, contentType, privateweb, privatemgr,
The rest of the tree display code will use the $pagetitle variable, but its value will be the menutitle from the query. There’s no need to change the variable name in all the places it occurs.