But in the petition before the trial court, all the evidence that was available in the grievance is also reviewed by the TC judge in a limited trial de novo. Say that he makes his own findings as to what happened (on a particular issue the hearing officer did not reach), which are different, but not inconsistent with what the quasi-judicial hearing officer held. Is a petition for writ of mandate not a "full and fair hearing on the merits"?
"Limited trial de novo" hardly equates with "full and fair hearing" to me. In addition, CE usually requires that the issue to be estopped is identical with the issue previously litigated. A writ of mandate (mandamus) is an extraordinary writ. At the trial court level, the court was facing the question of whether or not to compel an official (the quasi-judicial officer) to do something. So, while the evidence may have been viewed de novo, the review was done only with regard to how bad the officer screwed up. The issues are therefore not identical. In the first case the issue was, "Is the quasi-judicial finding so contrary to law that we should compel that officer to change his ruling." The issue that the man will now try to litigate is "Was I wrongfully terminated?"