Mickey is actually entering the public domain next year. Finally. However, Disney is already trying to get around this by trademarking the shit out of Steamboat Willie.

I highly doubt that if you created a work that infringed on someone copyrighted works that you could release it as public domain to skirt around the rights of the holder. An AI generated work - eligible for copyright or not - would likely have the same rules applied to any other works if it infringes someone elses copyrighted work. I don't see why a court would give up someones claim to copyright just because a infringing work entered the public domain.

Not aware if this has been tested in court before or not though - seems unlikely that someone would try this. AI work does mean this is more likely to become a case at some point though. But I suspect there are many other battles that will be fought first.