We've been using software to fix grammar for a long time, and AI does it also. The question is valid: if I get an LLM to fix a few grammar errors in my own writing, am I ripping anyone off? We can't just dismiss the question just because grammar fixing is something we did without machine-learning AI trained on vast numbers of other people's texts.

The output does depend on training works, even if you are just fixing grammar errors. But the document is obviously a derivative of your own writing and almost nothing else. A grammatic concept learned from vast numbers of worsk is probably not a copyright infringment.

Similarly, a part extraction concept learned from training sets such as pairs of mixed and unmixed music, and then applied to someone's own music to do accurate part extraction, does not seem like an infringing use. All features of the result are identifiable as coming from the original mixed audio; you cannot identify infringing passages in it added by the AI --- and if such a thing happened, it would be an unwanted artifact leading us to re-do the part extraction in some other way to avoid it.

The question doesn't feel legit to me though. The OP somehow found the one justifiable example among a sea of AI slop.

Justifiable because there were some filters. That may not even have been "AI". They may have been some deterministic algorithms that the software maker has to label "AI" because they otherwise think it won't sell...