I completed Leviathan on Prestige difficulty back in Season of Arrivals or Season of Undying can't remember but I don't have the prestige Legend of Acrius ornament unlocked. I still do not have Acrius, it's in the kiosk for me and I would like to get it and use that ornament on it but I don't have the ornament for some reason. I never finished the Acrius quest before the Leviathan raid went away but I did complete a Prestige run of it with others who were getting the catalyst. The ornament on the collections page says it's source is "Leviathan raid on Prestige difficulty". I did that so that means I should have this ornament despite not having the actual gun, like when you get an ornament from eververse for a gun you might not have. I'm sure its got to do with not having Acrius at the moment of completion but it doesn't say that anywhere on the page for it and I feel like I've been cheated out of this ornament despite fulfilling the stated requirements to get it.
The ornament was the last part of the acrius quest, even though you did the prestige raid, because you weren't on that stage of the quest you didn't unlock the ornament.