Those are the tributes that you need to collect Archon. You will find them there.
Well I can't see any other reason other than a bug. Cause it's the first time I hear about it. So please Archon submit a ticket to Plarium Support Center along with screenshots so they can investigate the issue even more.
It works for me, and I'm also at a bit of a loss why tribute grain would not be added to the store.
One (rather far-fetched) possibility: what exactly occurs with tributes when when consumption exceeds production and grain storage is down to zero? Is the tribute added to the store and then slowly ticks down to nil? I've never let my grain storage hit rock bottom when in the red with consumption so I don't really know, perhaps someone else does?