Bug 15347 notes that creating items via acquisition process doesn't populate 952$o via the itemcallnumber preference. It appears that item creation on MARC record import with embedded items similarly does not populate the 952$o via itemcallnumber. To recreate: stage a MARC record with partial 952 tag data, not including 952$o. Stage and import. You will have created items; none have call number populated. Open item editor and see that there is a value for the 952$o, but it doesn't come through until the item is saved. I can see not wanting to populate that field if data already exists, but if the field is empty, it is reasonable expectation that itemcallnumber should behave as it does when cataloging.