When a group is renamed it must be renamed in both minimized and maximized state.
Not sure if this a bug or intended functionality, it is a bug for me :)
It's a feature, of course. :-)
But you can have it your way (i.e., use the same text for both labels) by checking option Synchronize Open and Closed Group Label in the yEd preferences dialog (File > Preferences... > Editor tab, second from top).