My response was not an answer in reality it was a question.
As an answer to this question: difference between the delivery date control setting "None" and "Sales lead time"
None will, as standard, ignore the sales lead time defined in the default order or site specific settings. I have just configured this to verify this happens, I had 17 days set as the lead time, delivery date control set to none and the delivery date on the sales order line is today. "Sales lead time" uses the sales lead time.
If your system is defaulting the sales lead time onto the sales line with the delivery tab of the line showing the delivery date control as None then there is an issue. In mine I can change the None to Sales Lead time and the available ship and receipt dates prompt comes up warning me the date is prior to the expected date we could make this committment.
Any customisations in the area?