Issue:
I played Herigast, sacrificing a creature of an opponent I stole with opportunistic dragon. Using his ability I drew into Emrakul, which I was not able to cast even though I had means to produce to mana. (Emrakuls 11 - Herigasts 9 = 2 mana needed).
However, while trying to cast it, it told me to sacrifice an artifact as part of the emerge cost. I had Orb of Dragonkind and Ornithopter of Paradise on board, and they where apparently valid options, although too low mana cost to use with just 2 mana.
The turn after I tried casting Emrakul anew- the same problem arose, but I had enough mana to be able to emerge it from the Orb of Dragonkind.

1.  Reproduction:
   

   Play Opportunistic dragon
   Steal an opponents creature
   Play Herigast, sacrificing the stolen creature
   Try to cast a normal creature using emerge (Fails)
   Cast an artifact
   Try to cast a normal creature using emerge from artifact (Succeeds)

2.  I am playing on PC
   

3.   The actual results I experienced:
   

   Herigast gave my cards Emerge from Artifact

4.   The expected results of the feature:
   

   Herigast should give my cards Emerge