This has happened at least three different times. Initially I assumed I had somehow missed that the quest was to study two items not one, but this is definitely not the case now that I am watching for it. I leave my character studying while I am logged out and when I log in the item is complete - but the completion does not register and I have to get am additional example of the curio and study it all over again.
Items involved have been cat gold, and a seer's bowl. I don't remember what the third item was.
Possibly relevant: when I logged in I did not see the usual stack of little numbers showing all the points for the things that completed studying while I was logged out.