Apologies, @thomaseverest I thought I had commented about this one... Now I realize I had never put my partial thoughts out at all.

The species japonicum has often been considered only a local form of M. lacustre, so it would surprise me if it ended up in a different genus (or even subgenus).

It was not included in the 2023 analysis of Bespalaya et al, but it was included in Lee & Ó Foighill, 2003.
https://academic.oup.com/zoolinnean/article/137/2/245/2632282
In that paper, it was in Musculium with M. miyadii, M. lacustre, M. partumeium, M. securis, and M. argentinum.

That's what I think is best.