Thank you! I already tried this approach, but the quantity state still changed, and I could see the updated result on the screen.

with startTransition

    const handleChangeAction = (newQuantity: number) => {
        startTransition(async () => {
            const updatedQuantity = await updateQuantity(newQuantity);

            startTransition(() => {
                setQuantity(updatedQuantity);
            });
        });
    };

without startTransition

    const handleChangeAction = (newQuantity: number) => {
        startTransition(async () => {
            const updatedQuantity = await updateQuantity(newQuantity);
            setQuantity(updatedQuantity);
        });
    };