I know this is about the Japan and not the US, but software patent law has been incompatible with traditional IP protection in the US for a long time and it really doesn't make sense in the current age.
Software patent law and AI inventorship are separate legal issues. For example, an AI could invent a mechanical device which is a more "meat and potatoes" kind of IP. But the question of whether the AI, or the human invoking the AI, should be listed on the patent is distinct.