In GetItem, the key itype comes from items.itype and if not defined from biblioitems.itemtype. This code should be based on item-level_itypes syspref.
See Bug 9532 comments : http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=9532#c13
Created attachment 23878 [details] [review] Bug 11463 - GetItem must use item-level_itypes syspref
I've attached the patch but it surely needs a huge test plan.
This is already fixed, now that GetItem calls Koha::Items->find()->effective_itemtype