If Reinforce reduces a card's energy cost to 0, and that card is then returned to hand (via Pack of Wonders), does the cost reduction persist when playing it again from hand?

Ruling: No, the cost reduction does not persist. When a card changes zones to or from a non-board zone (like the hand), all temporary modifications including cost reductions cease to be tracked on it.

Sequence:

• Reinforce reduces the energy cost of a card
• The card is played with the reduced cost
• The card is returned to hand via Pack of Wonders
• The cost reduction is lost when the card enters the hand
• Playing the card again from hand requires paying its normal cost

Nuances:

• Cost reductions are only applied while you're actually paying for the card, not as a permanent modification to the card itself
• The hand is a private zone where individual card modifications cannot be tracked
• Cards changing zones become different copies and don't retain temporary effects like discounts, buffs, or damage