Conclusion: the reason mod-graphql doesn't get an instanceTypeId back from mod-search is that, unlike the old mod-inventory-storage request, it doesn't include that field (and some others) in its search-response summary records — only its full records.

When doing the initial search for inst000000000025, the Snapshot UI fetches

Which breaks down as:

And is on the same endpoint as the request that mod-graphql makes for the Z39.50 server.

And the result is:

Which is identical to what mod-graphql gets back.

By contrast, when the UI fetches the same record in the example given above, it does it by directly addressing the specific record rather than using a search:
https://folio-snapshot-okapi.dev.folio.org/inventory/instances/e54b1f4d-7d05-4b1a-9368-3c36b75d8ac6
That may be why it gets a more detailed record.

So here, mod-graphql's top-level query into mod-search is:

Which breaks down as:

And the result is a record that does not include the instanceTypeId field (or very much at all, actually):

I am now running the Z39.50 server with GraphQL side-loading, so that it contacts a local instance of mod-graphql rather than the one on snapshot. And I am running that local {{mod-graphql }} with URL and response logging turned on. Here's what the beginning of the log says: