Resolution: Not A Defect
Jenkins LTS 1.532.1
Flexible publisher plugin 0.12
Jenkins Subversion Tagging Plugin 1.16
For a matrix job you can add flexible publisher as post build action. Afeter that you can choose the wanted action of the flexible publisher.
Unfortunately all existing post build actions are listed there, including those which don't support matrix jobs, e.g. "Perform subversion tagging on successful build" of the Subversion tagging plugin.
Improvement: For matrix jobs filter out actions in flexible publishers action list, that don't support matrix builds.