Why do Letters vanish on App?

Great newspaper app, except for the often disappearing commentators letters!!!! I repetitively have to revert to the NYT websight to read those, even if the article is still up (or saved!) in the App. As even respected NYT journalists can not possible know all there is to know on a subject, readers comments/letters often "complete" the articles.

KLintheMoon about NYTimes – Breaking World, National & Politics News, v3.15.1