Player A plays Emrakul, the Promised end targeting Player B.
Player A takes control of player B during an extra turn.
Player under control casts Wish so player A can takes a look to player B sideboard which should not be possible.
I don't believe this is a bug, if you control your opponents turn you should be able to look into sideboard and cast the cards with wish. You are able to look even without casting Wish. It's like this in paper magic
This is the ruling set out by Wizard of the Coast for Emrakul, the Promised End regarding Sideboards.
Controlling a player doesn't allow you to look at that player's sideboard. If an effect instructs that player to choose a card from outside the game, you can't have that player choose any card.(2016-07-13)