Bug 29661 and bug 29662 highlight a general confusion about how PrefillItem and SubfieldsToUseWhenPrefill are expected to behave, which makes it too easy to inadvertently change behavior. More detail in the manual could be helpful. The two bits in question are whether or not fields should be prefilled when duplicating an item and whether a blank SubfieldsToUseWhenPrefill should mean all subfields are prefilled or no subfields are prefilled.
I amended the system preference on bug 29662 and we can use the new Needs Documenting after to make sure this goes into the manual. Marking duplicate.
*** This bug has been marked as a duplicate of bug 29662 ***