Animal Fair with Haunted Woods

[Timaaah]

Either there is a bug which incorrectly activates the Haunted Woods "top-decking attack" after Gaining an Animal Fair by Trashing an Action, or, the wording on Animal Fair should be changed to "Instead of paying this card's cost, you may buy it by trashing an Action card from your hand."  Or maybe "by trashing an Action card from your hand during the Buy phase".      I assume one of the latter, since trashing to gain an Animal Fair uses up a Buy.  (As written, you should also gain an Animal Fair when you trash a Treasure Map during the Action Phase.)

[Ingix]

The relevant part of Animal Fair reads:

Instead of paying this card's cost, you may trash an Action card from your hand.

You cannot pay a card's cost at any time just because you want to. In fact, the only time you pay a card's cost is when you are buying it. So what you suggested as a rewording is already true, the only time the quoted part is relevant is when you are in the process of buying Animal Fair.

That means what you did was buying an Animal Fair (not paying the card's cost but doing something else instead), so the game saw you buying a card, which correctly made opponent's Haunted Woods trigger.