You're right--Quest Items don't vanish with time, all others do.

The other notable items from that area (That I can recall offhand)
Gnasher (Club+2)
Spear of the Unicorn
Bracers of Archery
A handful of Fire Arrows

I don't think the bug is related to Cernd's shapeshifting. I've done the druid challenge with Cernd shapeshifted and had no problem. This might sound like a stupid question but are you sure you went to the right tent? The djinn guard should still be outside their tent when you return from the Grove.

I killed the Djinni's. /me does evil laugh