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.
> I’m asking because I visit Japan on a tourist visa and I have never encountered this rule. I’d like to learn more about it from official sources.
Unfortunately, I don't think this a specific rule which makes it hard to point to a source, but you simply aren't allowed to work in any form if you are in Japan on a tourist visa, and from the government's perspective, this obviously includes remote work.
If you want some specific government opinion in writing saying that remote work is illegal under tourist visas under current law, after the government holds public comment, it usually publishes an interim report compiling the comments it has received along with analysis, and I imagine that in this case the report will note this, but right now I can't find an official document stating this. However, googling it, there are various opinions from law firms agreeing that it is illegal.
There is also no reason you would have "encountered" this because the only way you would be able to encounter it is if you somehow made the government aware that you were illegally working remotely and they deported you.
It's basically impossible for them to tell if you are working remotely though, so it's almost a moot point in most cases unless you specifically go out of your way to make the government aware.
> you simply aren't allowed to work in any form if you are in Japan on a tourist visa
You will never get the Japanese government to say this, if "any form" includes incidental remote work. And "incidental" is undefined and they aren't going to want to be pinned down on that either.
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.