Being that the old oil was burn doesn't really matter seeing as you got most of it and put fresh atf in.....but the fact that there was metal shavings from the previous trans that is what's probably the cause of your issue. Not additive is going to clean out metal shavings if they have made their way into the valve body and channel plate. You may get lucky that over time they will work their way to a point where they pass through the filter and caught but chances are they will have done damage to the valve or bore by then.