Is "plagiarism" by judges a standard practice?

Some people are saying that it is standard practice for legal representatives of both sides to submit "proposed findings of fact and conclusions of law" briefs to the court and for the court to then write the opinion by cutting and pasting and mixing and matching different statements from these briefs. However, these briefs are not even mentioned in the national Federal Rules of Civil Procedure but are only mentioned in the local rules of the different federal district courts, and the local rules are of course not uniform. Some federal district courts might not mention these briefs at all. Also, I filed several appeals in the 9th circuit federal court of appeals and two appeals in the US Supreme Court and was neither required nor authorized to propose language for the those courts' opinions (reminder -- any gratuitous comment about the outcome of my lawsuits will be deleted as off-topic). And there are also state and local courts with their own rules. So how can this procedure of cutting and pasting from such briefs be a standard practice?

Here are some federal district court local rules about these briefs:

Middle District of Pennsylvania (where KItzmiller v. Dover was tried):

LR 48.2 Civil Trials, Trial Without a Jury
In a civil action tried without a jury, counsel shall file requests for findings of fact and conclusions of law with the pretrial memorandum. Additional requests may be made during the trial as to matters that could not have been reasonably anticipated before trial.(page 39)

So in the above court, the proposals for findings of fact and conclusions of law are required only before the trial and are optional afterwards.

Central District of California (headquartered in Los Angeles)

L.R. 52-1 Non-Jury Trial - Findings of Fact and Conclusions of Law
In any matter tried to the Court without a jury requiring findings of fact and conclusions of law, counsel for each party shall lodge and serve proposed findings of fact and conclusions of law at least five (5) court days before trial.

Here, the proposals are required only before the trial and no mention is made of allowing them afterward.

Of course, it seems that litigants should have some way of presenting written summaries in addition to oral summaries at the conclusion of the presentation of the testimony and evidence, so maybe these final "proposed findings of fact and conclusions of law" briefs are common even though they are often not required by court rules. However, it seems to me that it is only reasonable to expect that judges who use quotes or ideas from these briefs identify the sources, just as with any other kind of source.