A few days ago, Carsten Senger reported a problem with how River5 reads river files.
I verified that it's a real problem, and have a writeup and a fix I'm testing, and asking for review from JS devs.
The upshot is this. As River5 is starting up, if a feed has more than one new item, and the feed appears in more than one list, items will not appear in all the lists it belongs to.
The problem is more likely to show up in rivers for specific feeds, a relatively new feature. But it is a real problem and needs to be fixed.
Dave