Are you sure that there was no message about the items suddenly disappearing?

Ah, there is indeed such a message, and the items reappear in the shop - very cool. Unfortunately, the message is blocked by the inventory screen which comes up immediately after using normal drop, and the message goes away once you close the inventory screen. This means you aren't likely to see the message unless you are looking for it (the 'Don't drop that junk in my shop' message has the same visibility issue).