In Wild Ranked as Tyrande, I played Netherspite Historian & discovered Sleepy Dragon which went into hand, then played Drakonid Operative the same turn. The Operative battlecry did not proc, perhaps because there were no dragons but the Operative in hand before Sleepy
Do you have any screenshots? Or a link to the game on hsreplay?
Did your opponent still have a deck or were they already in fatigue?
Operative discovers a card in your opponent’s deck.
If your opponent has no deck remaining, there’s nothing to discover.