For me, a lot of dissociation happens (besides experiencing difficult emotions) when I feel invalidated or not understood. Is it possible that something similar is going on for you here? That your therapist not only failed to grasp the importance of the issue but also (maybe not willingly) dismissed your feelings and perspective on the matter. Which maybe triggered your dissociative response.

I had this (invalidation and being dismissed) happen more or less constantly to me when I was little so it's no surprise that experiencing something similar today might trigger some dissociation even though I might be able to handle it differently today...
Just a thought - feel free to dimiss it if not appropriate

With regard to T realizing what's going on for you (dissociating): My T had a suspicion (based on me having some spaced out look at times, or not being fully focused) and sometimes even asked "what's going on" "where are you now" etc but I didn't take her up on it. Only once when I had a really strong dissociative reaction and couldn't remember most of the session (plus I had those weird body experiences in session) I brought it up the next time and we started to talk about it. This was after about 2 years of therapy. She said that it took her quite a while to pick up on me dissociating because it was mostly subtle and somehow I was mostly able to carry on having some sort of "normal" conversation, even though inside everything was turned upside down. Which kind of indicates that I've been doing that for a long, long time, most of my life probably. So what's visible on the outside might not reflect what's going on inside of you...

All the best, c_r