I recently read a judgment in which the judge constructed arguments that normally one would have expected the prosecution to make. In fact, the defense in the case argued that the prosecutor failed to demonstrate their case by not arguing certain points that they should have. This seemed to create no problem for the judge who just "remedied" the situation by making the arguments that the prosecution should have made in her judgment.
My understanding is that judges are supposed to weigh the merits of the arguments put before them, not make new arguments to repair or strengthen the case for one of the sides. I thought that the judge's tendency to bolster the prosecution's case with such arguments smacked of judicial bias in favor of the prosecution.
Is it grounds for an appeal to complain of judicial bias if a judge makes a de novo argument in their judgement, assuming that argument would have been necessary for a conviction? If so, what are the relevant precedents in United States case law?
(PS is there a technical term for the rules concerning the "role" of the judge in a court, what they supposed to do, and not supposed to do?)