I can relate because I have done the same as you, with family — telling the truth, setting boundaries and confronting wrongdoing towards me (or related within the family system). It makes me the “Scapegoat” for not complying with (dysfunctional) family rules.
My family reacts the same exact way you described: pretending to listen, maybe agreeing, and doing so to influence unexpected surprise and appreciation for this rare reaction by relating to me with an open mind, not acting defensive, and conveying interest about my thoughts and feelings. And then…. comes the “switch”, like a game, the follow up to the “set up” serving as amusement by knowing that topic discussed was important to me, I am likely disarmed by their initial show of empathy. And, “obviously” I believe the issue waa something they wouldn’t forgot or blow off.
Yet that is exactly what they do. Forget about it immediately - by never following up, and blowing it off. Sending me the message that topic was actually not of any importance to them, and if I confront them about this — they act like they can’t recall the facts, the point, sometimes even contribute incorrect facts, to change the context (in their favor that eliminates any accountability). Ultimately, they never internalized the discussion the way they portrayed. The message, is reinforced — I don’t have relevance, they are in control and determine what is “reality”.
It is classic Narcissistic behavior and is emotional abuse.
I am no longer vulnerable to being disarmed as this (and other) behavior(s), all are, extremely predictable. It is a relationship killing disorder, that is self sabotaging to their own quality of life. A life of ongoing loss and emptiness. I try to remember this, and the experiences through time, makes it easier to detach.