The hierarchical menu widget isn't searchable in any flavor of InstantSearch, as only the root level values are available from the start. Other levels would require additional computation before a search could be performed.
If this searchable behavior is essential to your use case, one solution would be removing the hierarchy between the filters and showing multiple independent refinement lists.
You could even potentially customize your front-end to show the lower levels of refinement only if there's a refinement chosen on the first level (though this level of customization is out-of-scope for our Support team).