For some reason a toolbar-only command (= a command that is not included in any menu) must be visible for the assigned custom shortcut to work. This seems like a strange limitation. One of the reasons for assigning shortcuts can be that you do not want to display the toolbar.
So I wonder if this is intended behavior?
Test:
- Assign a shortcut to the Std_Part command.
- Only if the Structure toolbar is visible will the shortcut work.