I have this card for few days and I try to use it in every deck. It should work as weaker version of Etali, fishing a copy of the first card on your opponent library and giving it to you with half mana. But sometimes it doesn’t give me any card, and I am not with a full hand.
Someone else encountered the same problem?
//Edited Title - Tombstone
When it doesn’t give you a card, does your opponent have a full hand? If so, it’s the same bug that messes up Etali.
You just missed my same post about this… 
https://forums.505go.com/discussion/76197/thief-of-sanity-suffering-from-max-hand-issue#latest
Hi everyone, thanks for reporting this. I’ll go ahead and pass this up to the team for review.