I've been farming them for an hour, getting plenty of all the other drops, and many Weeping Abbess Mitres.
Am I just unlucky, or do these ones actually not drop the unmasked Abbess Mitre?
Here are all the armor pieces and tincts I used.
Herald of the Maw Mask with Oathbreaker tinct, Damarose's Garb with Harbinger tinct, Judge Cleric's Gauntlets with Dark Oath tinct, and unpainted Angel of the Void Trousers.
The armor is Descrier Guide Mask, Tancred's Armor, Calrath Guardsman Gauntlets, and Paladin Leggings, all colored with the Martyr tinct.
Now to run it back for a different build and ending.