Azure DevOps pipeline runs API does not return System variables

Question

Azure DevOps pipeline runs API does not return system variables to identify runs belong to a particular pull request. The API documentation has variables field but in the API response it's returning empty. In the pipelines build variables documentation, System variables (predefined) are expected to be set automatically should return in the API response so that we can get info like System.PullRequest.PullRequestId and System.PullRequest.SourceBranch to match run with a pull request.
Is there any reason why the variables field is empty for a pipeline's runs response or it might not include all the runs?

Answer 1

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