Some of the quests are glitched. I've had this happen to me a few times with different adventures. It is possible the object is missing from the treasure chest.
I made a video series with walkthroughs of every adventure (I might have missed some minor adventures but did all the tombs) and at the end I had to start a new save twice before I could get my sims into the Nectary tombs. In the end, the problem seemed to be I had them living in a private house. The only time it worked is when I got frustrated and left them at the base camp in France. So you can never always know for sure what is the reason for the failures. they just happen. In this case the adventure started off well but there was a point where they had to travel to Egypt and deliver something and find something else and each time they arrived in Egypt it dropped out and giving them the adventure using Master Controller didn't work as it still dropped out every time. It was so frustrating.
The one failure I suffered that was my fault was in another game years ago when my sim was doing the adventure with a helper second sim and that second sim collected the object they were looking for instead of my sim who had the adventure. That adventure was not able to be completed.
