I didn't play my warlock in season of the splicer beyond one time and I never got to completing the "decrypting the darkness" quest.
I have read that a supposed fix is to decrypt an umbral engram or two and it should sort itself. I've decrypted over 10 and it is still jacked up.
The quest says to go to the decoder and pick up the quest... no quest populates when I visit, nothing changes post decode of an engram. Because I cannot complete the quest... I cannot focus engrams at all. The reason I'm annoyed about this is that this week there's a seasonal quest to focus decrypt 6 of them... which means I cannot progress my Wayfinder as intended which potentially prevents me from accessing content... please fix the issue sometime this millennia.
This is a known issue Bungie is investigating.
[quote]The Decrypting the Darkness quest cannot be obtained from the Umbral Decoder, and access to it and the Prismatic Recaster cannot be unlocked.[/quote]