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.
1. Interesting. Should work, per the wording on the card.
2. Don't forget to give us the game number, so we can see if we can replicate the issue/problem.
I think I found the game madeoutofpeople is talking about. In that game, Harvest was played twice by them, but never revealed a Tunnel.