Failing to create leaderboard via the API

I'm getting the following error when attempting to create a leaderboard via the documented POST url.

Here is the full error response:

{ "errors" : [ { "id" : "xxxxxx", "status" : "400", "code" : "ENTITY_INVALID", "title" : "The request entity is not valid json", "detail" : "The request entity data could not be processed. Please ensure you are sending valid json.", "meta" : { "position" : { "row" : 1, "column" : 1 } } } ] }

Except what I'm sending IS valid JSON, emitted directly from JsonUtility and verified via JSONLint:

{"data":{"type":"gameCenterLeaderboards","attributes":{"defaultFormatter":"INTEGER","referenceName":"TEST_AUTO","vendorIdentifier":"TEST_AUTO","submissionType":"MOST_RECENT_SCORE","sortScoreType":"DESC","scoreRangeStart":"0","scoreRangeEnd":"20000","recurrenceStartDate":"2025-06-25T13:00:00Z","recurrenceDuration":"PT30M","recurrenceRule":"FREQ=HOURLY;INTERVAL=1"},"relationships":{"gameCenterDetail":{"data":{"type":"gameCenterDetails","id":"xxxxxx"}}}}}

This follows EXACTLY the pattern in the App Store Connect documentation. I've even tried sending the example JSON they give (swapping out the id and fixing the quotation marks) and the server rejects THAT as not being valid JSON.

What am I doing wrong? Thanks!

Failing to create leaderboard via the API
 
 
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