If you give your instances localizedFamilyName for Japanese but no localizedStyleName, all exported fonts will have the same style name “Regular” according to Apple’s font panel. They fuse into one item there, which effectively disables style selection.
This can be avoided by setting redundant custom parameters which define all Japanese style names the same as unlocalized ones, but tedious nonetheless. Probably when a localized family name is present but a corresponding style name is absent, unlocalized style names should be copied to it.
I’m using Glyphs version 2.6.4 (1283).