Can't apply Apple Store balance to any order

I received an Apple Store gift card a few months ago from Phobio when I traded in my old MacBook Pro. I redeemed the card, and it is in my Apple Wallet. I have confirmed via the Apple website that the card's PIN has the balance my Wallet says it has.

However, I am unable to actually use this balance, in either the Apple Store app or when trying to make an order from the browser. There is no option to add a gift card balance, and there is no indication that a balance is being applied to the order. Tapping "Buy with Pay" just asks for confirming the purchase against a credit card with the full balance.

I contacted someone at Apple via iMessage, but they were unable to find a solution.

How do I actually spend this balance? (and yes, it is an Apple Store card and not an iTunes/App card).