This happens with many different types of containers I’ve noticed. The same would happen if you create a control within a {form, container, tabbed container}, drag it outside of the {form, container, tabbed container} and delete. I think once a control is placed within a parent container there is no way to disassociate it with the parent. Thank goodness for CMD-Z Undo.
have you tried first toggling the desktop display for that old container on and off, then renaming the old container, and then deleting it? this has worked for me as a workedaround to this issue