I submitted a bug report on this in game, but after coming back from a mission, Rinor (unique skunk follower from the comic) was traumatized and I needed to head to Anchordeep to get crystals to heal them.
Upon returning, Rinor was just...gone. Not in the follower list, not selectable for any ritual or clothing, not even shown as dead for resurrection. They just ceased existing.
I don't know of a way to get them back and now I'm worried for the likes of Narinder or the bishops also vanishing on me post crusade, just lost forever for no reason.