Without the feed URL it's a bit hard to troubleshoot this but Feedbro 4.5.0 now properly handles XML feeds that are in "oldest first" order which might have caused this in certain situations.

Edit (2020-07-29): another thing we noticed (and is fixed in 4.5.1 which is soon available) is that Firefox has a bug in Date object parsing when the year is expressed with just two digits. For example this string "Mon, 31 Aug 20 13:00:28 +0200" was parsed by Firefox JavaScript Date object as Wed Aug 20 2031 14:00:28 GMT+0300. This worked fine on Chromium-based browsers which made it hard to spot. This is also a likely reason for those skipped articles.

Of course, it would be have been a lot quicker to track down with a feed URL to test with. So for everyone else as well: if you think there's a bug somewhere, please include a feed URL that we can test it with.