That is bizarre. No student has the ability to delete another student's replies on someone else's thread. The only way that could happen is if you replied to a thread, and the student who originally posted the thread deleted the whole thing; the original post and all of its replies. Could that have been what happened?
The tricky part is that there is no reply for me to research because it potentially could have been deleted. If you feel like running a little test, we might be able to track it down. As you reply to other student's threads, jot down the URL to your replies. If one goes missing, we now have the link to it and I'll be able to track down what happened to it. It's not a fun test, but I'm out of ideas and really want to help.