I'm not sure if there's a specific rule that says that explicitly, but you aren't allowed to work if you are in Japan on a tourist visa, and the government's opinion is that this includes remote work for a company outside of japan, so it doesn't really matter that there's no rule saying that explicitly.
That's also the whole reason for potentially introducing this new type of visa; if you were allowed to do remote work on a tourist visa, they wouldn't need to do this.
That's also the whole reason for potentially introducing this new type of visa; if you were allowed to do remote work on a tourist visa, they wouldn't need to do this.