Yeah Yakuza 6 was the first noticeable FW drop in Japan, at about 220k~ when Yakuza 0, 4 and 5 all sold about 330-360k FW
From what I remember the Chinese release made up for the poor FW in Japan. It could be Yakuza burnout but we won't know for a minute.
That's probably it. With games being released yearly since 2005, I suppose it's to be expected that players might get a little burned out now and again.