Strange bug that I can't figure out the trigger for -- at some point the companions have blank inventories. The items come back at random times and go away again.
I've noticed this too. For me it was Gale, specifically, whose inventory ceased to exist apparently randomly. So far my best hypothesis is that it is a failure related to what's supposed to be a functionality where you can't access your companions' inventories if they don't like you very much, or something? I also noticed that Astarion's inventory was greyed out (differently from the blank version that is probably unintended, but suspiciously similar) for the first short while after I recruited him and it occurred to me to wonder whether the two might be related.