and displayed no emotion whatsoever. That was reinforced in the scene by the weirwood tree.

At this point, I think Bran is suffering from some sort of PTSD... lack of emotion may be what he needed to adopt to maintain his sanity from seeing all the presumably absurd shit that he sees on the regular. In other words, to do his job, he needs to divest himself from being emotionally attached to what he's watching.

Complimenting Sansa's wedding dress can then be interpreted as him showing affection towards his sister but also disclosing that he knows what happened that night.