This is by design - discount from coupon and basic discount are separate things, since they're applied separately. And discount filter searches only for basic discount, so it will hide all products without basic discount even if they have active coupon.
In your case I suggest to just disable discount filter. In general discount as a metric is quite unreliable - it could be manipulated by store (if the store inflates the base price, it could have higher discount than cheaper offers) and some stores do not have discounts at all, because they do not show base prices, even if there is a promo for specific product.
