The reason you see updated called is that for morphdom (which LiveView uses under the hood), the stream element actually changed. When the LiveView renders, the HTML morphdom sees actually does NOT contain the stream elements, because streams are reset after each render. This means that morphdom actually considers the element to be updated and wants to remove the child elements. When it tries to remove them though, LiveView steps in and aborts the removal: phoenix_live_view/assets/js/phoenix_live_view/dom_patch.js at b304ad5e97bfa98d4907afb79f503ccc5dba3cce · phoenixframework/phoenix_live_view · GitHub

You could use a MutationObserver in your hook:

Hooks.FilterUpdateDataHook: {
  mounted() {
    this.observer = new MutationObserver((changes) => {
      // something changed
    });
    this.observer.observe(this.el, { subtree: true, childList: true });
  },
  destroyed() {
    this.observer.disconnect();
  }
}