Continuing the discussion from Reply as linked topic no longer quotes original post:
(No, this isn’t a contradiction; I constructed that quote manually)
When you highlight text in a closed topic, and click “Quote” or “Reply as linked topic,” it just gives you the “Continuing the discussion from…” text; it doesn’t quote whatever you highlighted.
…And while testing this, I ran across another bug… If you have a “Reply as linked topic” composition window already open, and you open another Discourse window and try to do a “Reply as linked topic” in that topic, the new composition window will populate with the “Continuing the discussion from…” from the topic you’re in, and then append whatever is in the other composition window.
The “not quoting highlighted text in closed topics” is happening in both Chrome mobile on Android Lollipop, and Chrome desktop on Windows 10.