
This is expected behavior in DDMRP. Setting an item as a decoupling point does not automatically reduce the lead time to 0 for purchased items. The system continues to consider the purchase lead time because external procurement cannot be decoupled in the same way as in-house production. For production items, the decoupling point breaks the production chain, so the calculated lead time becomes 0.
Regarding the transfer lead time, D365FO uses the lead time based on the planned/default order type. If the item is planned as a Purchase order, the transfer lead time is generally ignored. It is only considered when planning uses Transfer orders. Verify the planned order type and coverage settings for the item.