Most of those ones i listed up top don't really fit into the whole Alt R&B label. Most would be under Future R&B, Future Soul, Neo Soul & Classic. I don't think i've really run into any Alt R&B from Japan tbh.
You might like starRo if you want something similiar to Alt R&B.
He's more of a beatmaker though with features & his sound leans more towards the whole Future R&B sound that you can find on Soulection. Listen to his album "Monday"
I would also recommend checking out Metome who is another beatmaker who leans more IDM but with Jazz & R&B influences. If you like Kelela's Take Me Apart maybe his album's Objet & Opus Cloud will scratch that itch.
https://metome.bandcamp.com/album/opus-cloud
https://metome.bandcamp.com/album/objet