I've seen people say they heard the final goodbye/I love yous, but couldn't trigger it on a reload. (I didn't have this problem on a PC.)
You will know this is coming when you've had the cutscene with the advisors and your hand flares up uncontrollably and they watch helplessly. It will be night time at the Winter Palace with fireworks going. THAT is when you go through the eluvian for the last time and someone in your group will say, "Leliana told us what happened with your hand." And then the LI in your group will comment on a variation of "don't tell me I'm going to lose you." This is the final goodbye dialogue.
(I don't know if Cullen or Josephine have a final goodbye for the LIs. Haven't seen videos on those two yet.)