Idk if it is necessarily a bias towards Japanese games as much as it is they have a different set of expectations that match the Japanese market. For instance, in a lot of jrpg reviews they talk about a classic Japanese "open field" feeling that they like, kind of what you get in xenoblade.

I am sure they would give a good score to a western game that catered well to their criteria, but Japan is a smaller market, so idk why a western company would try to do that. I'm not sure famitsu, or if japanese companies in general even have a great handle in what would be popular anyway - Ghost of Tsushima was a bit hit in Japan, and you have peole like Toshihiro Nagoshi lamenting that they couldn't make a game featuring a non conventionally attractive lead at a large Japanese game studio.