Answer
This has been confirmed as a known Boomi REST Client Connector behavior: the connector can send a body along with a GET-style request (which most tools, including Postman, don't do by default), and if the API doesn't expect a body on that call, this causes exactly this kind of Bad Request. Clear the document path with a blank message step before the connector (or route the connector onto a branch that carries no document content) so no incidental body gets attached to the request — this is a Boomi-connector-behavior fix, not a sign that the IFS API call itself or its bound-parameter setup is wrong.