My understanding is that it happens (or is supposed to happen) if you go to Morrigan's spot in Skyhold in the brief period after meeting Hawke's "Warden contact" but before going on to Adamant (provided you'd already completed the Wicked Eyes & Wicked Hearts plot first).
I remember seeing this in the days leading up to release -- Brianne tested it out, as I recall -- so I'd be surprised if it'd been cut. Less surprised if there's some other issue involved, however.
I'd heard that this was possible, so specifically tried for it in my current playthrough. I got Alistair first, then did the Ball, but never got anything and finally gave up and did Adamant. When Alister left afterward, he did tell me to tell Morrigan something for him, so it felt like there should have been some interaction.
Is it possible it only triggers if you completely ignore the Warden stuff before doing the Ball and getting Morrigan?