ATP and the MRP will consider the confirmed ship date - are the items available at the time of shipping. But the reservation, when you release to Warehouse, considers the confirmed receipt date, based on Transportdays and Sellable days.

This will conflict, as an Item can be OK, based on Best before date when shipping - but have exceeded Best before when it arrives at the customer.

I suggest a parameter, where you can Release to warehouse based on either Shipdate or Receivedate - That would solve the problem.