This is not the bug, it’s how the fixed fee added to cart as cart fee. The fixed fee means the fee without effected by quantity. And if we add this to sub-total as you mentioned in your screenshot then it will not be the fixed fee.
And we don’t have any other way to show/add price other than extra cart charges. If you need to add extra fees in cart item subtotal than disable the ‘Fixed Fee’ option.
And if you need to add extra fees into sub-total than we offer customization services for such price calculations. contact firstname.lastname@example.org for details.