Notice that with the \stopStaff and \startStaff commented out, there is a single StaffSymbol grob in the example. It makes sense that any override will apply to the whole object. When \stopStaff and \startStaff are in effect, a second StaffSymbol grob is created, and it can of course take a new override or revert.
Very surprisingly, there is only a single LedgerLineSpanner per StaffSymbol object! This means that any override of LedgerLineSpanner will affect all of them.