You are ignoring UserX. You have enabled 2-way ignore (hiding posts both ways) .
You start a thread in the D&D forum. UserX will see that thread when they view the D&D forum as if it were a normal thread. UserX will see your first post in the thread. They can respond to it, and other people in the thread. They will not see subsequent posts by you in the thread.
UserX starts a thread in the D&D forum. You will not see that thread when you view the D&D forum, and it is likely you don't know the thread exists. You will still see that thread if it becomes the most recent post on the Forum List, and a few other places. You can still post in that thread if you navigate to it. You will still get notifications about that thread if you follow it or someone mentions you in it. UserX will not see your posts in that thread. You will not see posts by UserX in that thread (including the first post).
With this form, no one is completely blocked out of a thread and unable to respond to it due to ignore settings. This is the way things work with the current forum software, but it's not how it has always worked in the past.
Hope this clears things up.