Jace, Telepath Unbound won't let me cast split cards in GY for their chosen cost

[duelcastermage]

With the flipside planeswalker of [[Jace, Vryn's Prodigy]] (Jace, Telepath Unbound), I -3'd and targeted [[Fire // Ice]] in my graveyard with two mana open. I never got to cast either half of the card; I think XMage assumed its mana value was 4 and I couldn't cast anything(?)

[github-actions[bot]]

Jace, Vryn's Prodigy // Jace, Telepath Unbound - (Gatherer) (Scryfall) (EDHREC)

{1}{U} Legendary Creature — Human Wizard 0/2 {T}: Draw a card, then discard a card. If there are five or more cards in your graveyard, exile Jace, Vryn's Prodigy, then return him to the battlefield transformed under his owner's control. :arrows_counterclockwise: Legendary Planeswalker — Jace 5 +1: Up to one target creature gets -2/-0 until your next turn. −3: You may cast target instant or sorcery card from your graveyard this turn. If that spell would be put into your graveyard, exile it instead. −9: You get an emblem with "Whenever you cast a spell, target opponent mills five cards."

Fire // Ice - (Gatherer) (Scryfall) (EDHREC)

{1}{R} Instant Fire deals 2 damage divided as you choose among one or two targets. :arrows_counterclockwise: {1}{U} Instant Tap target permanent. Draw a card.

[xenohedron]

duplicate of #9607