The automated FAST generation in OCLC relies on the subject headings being correct. This is not a special issue with the term Commentaries; it is a problem with every subject heading which is incorrect or invalid. As you point out, the system cannot tell whether a wide range of subdivisions are used correctly. There is no good reason to single out the FAST term Commentaries for special attention and work-arounds for correction. If there is a way to detect that a subdivision is used incorrectly, then the proper solution is not to have a work-around fix for the FAST terms--the proper solution is to mark the subdivision usage itself as invalid. If that cannot be done in a practical way, then there is also no practical way to detect it for the FAST generation.
I need to point this out because lately I have been seeing “655 #7 Commentaries. ‡2 fast ‡0 (OCoLC)fst01423723” generated for faulty subject headings everywhere, after names (600), name/titles (600), secular literary works (630), non-literary works (630). Or has there been a policy change on the use of “$v Commentaries” recently that I am unaware of?
Under SHM H1188 [Sacred works], there is an A-Z list of subdivisions that can be assigned. This is the only place we find the form subdivision $v Commentaries. That is to say, “$v Commentaries” can only be used for sacred works entered under the title in the bib (630 0 [Title]). For secular literary works (whether entered under title or author), there is a different list (H1155.8). Neither “$v Commentaries” nor “$x Criticism and interpretation” can be used for a work that is a commentary on such works.
Now, It’s understandable that the machine cannot tell if “630 00 Anacreontea ‡v Commentaries” is valid or not. But what about “600 00 Aristotle $v Commentaries” or “Aristotle. $t Poetics. $v Commentaries”? Would it be too difficult for OCLC to catch this type of mistakes and not to generate “655 #7 Commentaries. ‡2 fast ‡0 (OCoLC)fst01423723” automatically? Because, by definition, this form subdivision applies only to sacred works.
Just an observation.