In 21.05.07, the PreFillItem syspref is applied when duplicating an item. This was not the case in previous versions of Koha and is not the case on master.
To recreate (on 21.05):
- set PrefillItem to "the new item is prefilled ..."
- set SubfieldsToUseWhenPrefill to 3 so that the Materials Specified value is prefilled
- create an item that has a Materials Specified note
- confirm that when the Add Item page reloads after creating your item, the Materials Specified note is prefilled as expected
- find an item without a Materials Specified value, duplicate it
- confirm your duplicate item's Materials Specified is prefilled with the value from your first item, when it should be null like your second item
I think that we've noticed the same thing in 21.05.x and similar report that it didn't happen in the past.