This is interesting because I could have sworn we fixed this issue a long time ago. In fact, I just searched the logs and can confirm that Reeder is correctly using the API so that this issue shouldn't present itself.
Although I think I'm reading this wrong. Are you only talking about newsletters? Those are built the same way as feeds. To a NewsBlur client like Reeder, there should be absolutely no difference between a normal feed and a newsletter feed. The difference is in the input, not the output. So I'm not sure why that would be the case.
Yes, it only affects newsletters. In summary, if I don't use Newsblur web site for a few days, I'll stop getting the newsletter in Reeder. I open NB website and I'll start getting the newsletters, including the 'old' one I didn't get before.
Not sure how I can help but if you think of something, let me know. If not, feel free to list my new items. I logged to the site today so you'll see the newsletters but wait few days and they won't show.