Not many people know much about R&B in Japan, partly to do with the fact that it not too much of a popular genre here. Even the music they call R&B tends to sound very pop-like. Generally they will class the music from half black Japanese as R&B even though it is basically pop. E.g. Crystal Kay, Teruma Aoyama or Ai.
I do recall however some Mary J Blidge sounding R&B by group called Double. The group originally consisted of two twin sisters hence the name double, however since one sister passed away, the remaining sister Takako Hirasawa continued a solo career in the same name. The debut album was considered the major major success for modern R&B in Japan.
It is been said that this has paved the way for other successful artists as mentioned above. However I have not heard much real R&B from Japan since then apart from Nao Yoshioka. There seems to be a large gap in the market there or record label simple are not willing to take risk with this genre. Check out the below video.
In terms of production, many Japanese musician do have a high level of theory and understanding when it comes to R&B and soul. However most had not grown up around this genre and may find it difficult to improvise. But if you go into a studio in Japan with concrete plan, keys, phrases and inform the musicians beforehand, session will run very smoothly. Deviating from the plan can cause issues, but that will often depend on the musician. You can already count on the Japanese to perfect their parts in the studio in few rehearsals. It is normally us westerners that require multiple takes due to the fact that like mixed things up and natural improvise a little more.