Same issue occurring for me. I bought/installed both Seeker, Slayer, Survivor and Forgotten Sanctum at the same time, but while I got a trigger for SSS within a short time after finishing Ashen Maw, nothing I do seems to trigger the FS event chain to start (I haven't even got the message that some are referencing about an 'earthquake' or something like that). I have tested on both a game that was pre Ashen Maw (and got the SSS event chain to start after I completed it) and a pre-endgame save that had already finished the Ashen Maw quest; neither started.
Not sure if this has to do with adding these DLCs simultaneously or what, but it's definitely a bug and not just not understanding how to trigger the quest chain. Any help appreciated as right now the DLC is effectively 'off' for me.
