What I understand is that Rob was the first to pull away from the group, due to Sylvian. Rob got tried of being bossed around and was starting to do session work for some big names. Plus he started another band in Australia not long after Tin Drum was released and then in the 90's had a band called "Slow Club" He also rehooked up with Jansen, Barbieri and Karn on "Beginning to Melt" and a couple other tracks in the early 90's.... other than that He really had nothing to do with Japan or Sylvian after "Polariods" even came out....
This is all well documented information. I actually emailed and spoke with one of Rob's personal friends on Mickkarn.net.... if you want You can research all the posts by "bleeper" I'm guessing a fellow ornithologist and co-worker of Rob's.
Check out the supposed lost Tapes of Rob Dean and few Japan pieces.
http://a8creative.co.uk/orpheus/robdean.htm