Have noticed this on a few occasions now.
Add item to download list from available subjects (Is known to be incomplete)
Item is sitting in download as incomplete, still waiting further parts.
On groups refresh, the item gets marked as old in the subjects, and as Hide Old is selected, now dissapears from view. You now have to unhide old, and find the item in question, so you can add the remaining parts when available.
Not sure if this is a bug or by design, I suspect the former, though if it is the latter a preferred protocol may be handy