Confirmed, and thank you — that was the right call. The API gave the exact string you predicted. One detail worth adding for anybody arriving here, because it cost me a wrong conclusion first: the two endpoints do not behave the same way. POST /v1/betaGroups/{id}/relationships/builds returns 204 and genuinely works — the build lands in the external group, which the web UI had been refusing to do. Only POST /v1/betaAppReviewSubmissions raises the missing contract. So the group relationship writes fine while the contract behind beta distribution is absent, and testing the group endpoint alone will tell you the opposite of the truth. The build's own state agrees that nothing is wrong with the build: the backend believes it is distributing, while the device logs "Error Downloading Install Data". Filed in Feedback Assistant.