What I'm seeing is quite a lot of people not quite understanding the difference between the 432 and the 432-R. I think that will probably continue, just as it does in the classic Porsche world where Mr Average just won't have a deep enough interest to spot the difference between a 911S and a 911R at ten paces.
I take your point about people tending to think that the rising tide raises all ships, but that's been happening across the rest of the S30 range with people wanting to believe that recent sale prices for early/'significant' 240Zs are pulling up prices of later 240Zs, 260Zs and 280Zs. They may even be right to some degree, but there are still a lot of those cars extant and available, which must suppress average values somewhat.
That's not the case with extant 432s and - more so - the 432-Rs, most of which are known to enthusiasts in Japan individually (by chassis number no less), and there are not many hiding undiscovered any more. This will tend to keep prices high.
I think it's fair to say that rising values of 432s and 432-Rs - along with other rare models like the 240ZG - have been of concern to long-term owners in Japan. People who bought their cars 20, 30 and 40 years ago are not necessarily rejoicing, and some may see high value as something of a burden due to unwanted attention (the wrong kind of attention...) and higher insurance implications with strictures on storage and use.