I'm not an administrative law expert, but the way a mistrial works, in terms of the deference courts may give to tribunals, is linked to the expertise of a tribunal. Let's say it's the copyright context. It doesn't mean the decisions themselves are always followed. They're definitely not, but at least the starting point is that they recognize there has been expertise brought to bear.
Right now, however, even though I think we would all acknowledge the Privacy Commission has expertise, it's not viewed as an independent tribunal. Thus the courts have taken the position that they have the ability to start looking at a case from the beginning.