@SASSBS: Huh. If you can reproduce this with another collection, a backup of the collections.json db *before* the interaction with the plugin might shed some light on this... (If you haven't used the plugin since, the last backup it did might be enough, I'm a bit fuzzy on exactly how/when the backups are done).
@Raederle: If I understood you correctly, you're speaking of periodicals/news feed? If so, the Kindle *should* keep at least a small amount of back-issues (~10, maybe less). No idea how well it handles things if you feed it only 'old' backlogged items.
Regardless, I don't think we have a way to get our fingers in the Kindle's cataloging system (at least from here), so, the point is moot anyway