IDK if this has been discovered or mentioned yet, but once you do the event for shuddering hollow, regardless of whether you got the citrine or not, the respective seller for the citrine will sell them to you.
To shed some light on this I logged in, killed slahnarok and got the Stormbringer’s Runed citrine. From that point forward I kept killing the boss to try and get the Legendary Skipper’s citrine, but the event would bug out and not give me credit for it. Well, out of bordem and curiostiy, I was messing around with the vendors to look at the cosmetics they had, and noticed that Taljor was now selling the Legendary Skipper’s Citrine, but Angorla was not selling the Fathomdweller’s Runed Citrine. Yet when I bought the skipper citrine, it automatically changed the shuddering hollow reward citrine from the skipper citirne, to the Fathomdweller’s citrine. I was awared the Fathomdweller citiren on my next kill, probably out of luck. Yet I think the solution if you are strugging to get the citrines from shuddering hollow is to do as follow:
Kill the boss normally, and if it bugs out, buy the citirne from the respective vendor given it seems to still be awarding credit for unlocking the ability to buy them after the fact. Then rinse and repeat as needed until you get all the ones you need!