I discarded my terisian mindbreaker with Jace Reawakend. It showed as castable next to my hand cards that same turn (but I didn't have mana to do this).
Next turn when I wanted to unearth it the mindbreaker was in the exiled pile (and uncastable).

Since my win condition was unearthing the mindbreaker and attacking and then the next turn exhausting riverchurn monument this cost me my win.

Please check what went wrong. I cannot see anyway how that card could have gotten exiled.