gift article (link work??)
https://giftarticle.ft.com/giftarticle/actions/redeem/a8e570d5-71be-4068-b4b0-c9ca650f3b2c
That's a pretty complete paywall.
https://japantoday.com/category/business/japan%E2%80%99s-newest-shinkansen-has-no-seats%E2%80%A6or-passengers
Says that it's not (only?) seats going unfilled in the trains, but a shortage of truck drivers that pushed East Japan Railway to do this. But we can assume that an EJR PR piece would not highlight low ridership