When tagging a build using the Subversion SCM, if you elect to choose the Use atlernate credential for tagging option, any alternate credentials specified are ignored, and the tagging proceeds using the same effective user that Jenkins normally uses for accessing that target repository.

This issue was touched upon before in JENKINS-2053, in the comment by idlysphere on 14/Feb/2010, but I think it was missed because his comment was not directly related to the original focus of that ticket.