If a standard free-sytle downstream job is fingerprinting a file it is correctly identified as downstream build. But for a downstream matrix job this does not work. This is even though the fingerprinting works correctly as seen by fingerprint details. The fingerprint shows each of the configurations of the matrix job as a user of the file.
This issue has been described before at http://jenkins.361315.n4.nabble.com/Question-about-Matrix-project-amp-fingerprints-amp-Join-plugin-td3857309.html