I apologise if this is a repost, i used search but just found super old posts.
Anyways, i server xfered hoping that it would reset the content of all my hallowed end bags (it didnt btw) but i noticed something very strange when i went in to zg to complete my fullclear i was too lazy to do earlier.
Although the game counts me as saved for boss kills in quite a few instances... after I xfered i was able to zone into instances and the "fresh" instance over-wrote the partial clear in terms of saves.. it added the other bosses to my previously cleared list... but i was able to kill mandokir all over again.
Not 100% sure how helpful this is to everybody, but anytime i find something i try to share it.