I’ve run some tests as suggested but I’m getting inconsistent results. When the coupon value meets or exceeds the product price it’s working as expected, but it’s also sporadically reducing the price to zero when the coupon price is lower than the item, for example product at £10 applying £9 coupon gave it for free, and also product price £55 with £33 coupon also gave for free.
So whilst I can’t be specific about why this is failing, this doesn’t appear to be a good build.