It kind of is both, the issue is caused by some CSS that becomes useless after it uses li.bbp-body that seems to not close properly, breaking the markup when threaded replies is enabled. The avatars going to the top left corner is caused by absolutely positioning the avatar when it reaches about 480px.
https://bbpress.trac.wordpress.org/ticket/2823
A possible workaround for this would be to have a horizontal forum layout.
This stylesheet has an example of the horizontal layout I was talking about. You can Place the file in a folder called css in your child theme to see how the layout takes affect. I have not tried it with 2016 theme yet though so I do not know how it will do.
https://gist.github.com/robkk/ec3129ef78397783fa2a23389be47649
