POS Cart UI Extension - setLineItemDiscount stopped working

[peterasmith]

I have a custom POS UI App which adds a line item discount to the cart.

It recently stopped working, and I can't figure out why. It's not throwing any errors, but the discount just doesn't apply to the cart any more.

Code looks like this

api.cart.setLineItemDiscount(lineItem.uuid, 'FixedAmount', 'Offer Title', '10').then(()=>{
    api.toast.show('Discount applied');
});

I'm using

"@shopify/app": "3.58.2",
"@shopify/cli": "^3.66.1",
"@shopify/ui-extensions-react": "^2025.1.3"

I've tested the cart level discount - and that still works correctly. It seems like it's only the line item discounts that are affected.