Yes, the drop rate of the belts isn't very high I guess. I have done Vilithrax a few more times to get the belt after all but I didn't get it once. Though it almost always was a "Great Success". And from now on I will always take a screenshot after getting a rare item to make sure, that I have proof. xD
The same happened to me 20 minutes ago. I have slayed Vilithrax and got the Belt of Wisdom but I didn't find it in the inventory. But I wasn't prepared for that bug and didn't take a screenshot before I have changed the instance. I reported it as a bug. It is a pity, because I was very happy about finding the belt -_-