Tunnels should fire on being discarded; Harvest says "reveal the top 4 cards of your deck, then discard them."
Therefore, if a Harvest reveals a Tunnel, it should fire the on-discard event and so offer a gold. That doesn't seem to be happening -- much to my disappointment last game.
Therefore, if a Harvest reveals a Tunnel, it should fire the on-discard event and so offer a gold. That doesn't seem to be happening -- much to my disappointment last game.