Guest API: 401 on POST /api/v2/webmessaging/conversations/{id}/close

Getting a 401 Unauthorized when trying to close a web messaging session from our backend. We have a valid OAuth token with webmessaging:conversation:close scope. The endpoint is POST /api/v2/webmessaging/conversations/{conversationId}/close. Payload is just {}. Works fine in Postman with the same token.

In we’re using a action to call this REST endpoint. The token is refreshed every 55 minutes. Is the token valid for this specific Guest API scope? Or is there a header missing in the REST Proxy configuration?