I am also having this problem on PS4. I have completed One Less Venatori, never said anything in conversation that leads to Dorian's disapproval, and just finished the Winter Palace with all the options that lead to "Dorian Greatly Approves," and he still won't talk to me about anything except questions regarding Tevinter.
I've heard this problem can be caused if you have too much approval with Dorian prior to going to Skyhold, but this is unconfirmed at the moment. This would also mean the bug only occurs to players who sided with the mages, which I doubt.
I did side with the mages, so theoretically that's possible in my case.
Community AdminAye, one issue with the hidden approval ratings is that you can't immediately check approval level for confirmation.
I have seen the theory about the early high approval too, it sounds plausible.
