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The conversation in his office takes place some time before his discussion with Cassandra. If you continue questing, I think everything should be fine.

The discussion with Cassandra will happen after you do either Here Lise The Abyss or Wicked Eyes, Wicked hearts. Make sure you finish that and speak to Cullen in his office after talking to Cassandra BEFORE you talk to Sera as she may start her prank quest and, if Cullen's is not done, he can vanish form the game for the rest of that playthrough.

I think that's normal. I always got the first lyrium addiction talk quite some time before the discussion with Cassandra.

First he tells you that he stopped taking Lyrium, that's the one you got. Next lyrium cut scene is the one with Cassandra, then talking to him after and deciding if he takes lyrium again or not. That's how it's supposed to be.

@danichann Just remember to save (manually) often, especially before character quests, just in case the game decides to be buggy 😉 And enjoy 🙂