The `Query Entities Via POST` keyword (and relatives) send `Content-Type: application/ld+json` **with the `@context` in the Link header instead of in the body**. Per TS 104-176 § 6.2.4 the two are mutually exclusive: with `application/ld+json` the `@context` *shall* be in the payload body and *shall not* be a Link header (the Link form is only valid with `application/json`). The broker correctly rejects 400 BadRequestData. Affected: `D011_02_exc_02 / D011_02_red_02` (and the family). Fix: when the content-type is ld+json, inline `@context` into the body and drop the Link header — or switch the two TCs to `application/json` (as the GET variants already do).