Yes, congregation numbers show greater decline than publisher numbers, which is probably a fairer reflection of decline as experienced by JWs in Japan.
I wonder if the end of JW growth in Japan coincides with a broader disillusionment with American influence in Japanese culture in general. I don’t know much about Japan except that they experienced economic problems since the early 1990s and turned to a more nationalistic orientation as a result.
If Japan is experiences secularisation in any way similar to that in western countries in recent decades then the decline of JWs might be part of broader secularisation. Like in many western countries it may be the case that JWs are declining at a slower rate than other groups.