We define mutating imports to be an error, however the standard currently says nothing about what happens when an exported binding is mutated from within the library where it's defined.
In many common library implementations there will be no effect (i.e. the import effectively gets a copy of the original), whereas in a namespace based implementation the change will be reflected, so a conservative approach is to add a note saying the result is unspecified.
The WG voted to reject this proposal and leave the validity of mutating exports unspecified.