That is not intuitive IMO, and users have complained. I'd call it a usability bug. Another one is the receiver list not updating, when someone leaves the thread. Surely that is a bug?
I don't need support. I'm trying to help you improve your product. You're welcome. :)
Xenforo 1.1.4, localized Finnish. Due to localization, I'm unable to provide exact English names for options. This is reproducible every time.
Steps to reproduce bug:
Start a private conversation with some members
One of them removes herself from thread...
...Choosing not to receive further...