Found it. NewsBlur trims a feed nobody has read in a few weeks down to its 10 newest stories. IEEE posts each issue as about 50 items at once, and the front matter (Table of Contents, society pages) has the newest date, so the trim kept those and threw away most of the articles. The IEEE feeds you read often kept everything, which is why it looked inconsistent. That’s fixed and will be live in the next deploy, and the missing articles come back the next time IEEE updates each feed.
One more thing: IEEE dates each article by when it first went online, often weeks before the issue. Many arrive older than Premium’s 30 day unread window, so they show up as already read. Premium Archive keeps them unread.