BuddyPress.org

What theme are you using? It’s possible that this problem is specific to the theme. Try on a default WordPress theme like Twenty Sixteen to be sure.

Can you share a screenshot of the “comment box” and the “reply box”? It’s unclear from your questions whether you’re talking about textareas or about buttons. Also, BP doesn’t have separate Comment and Reply functionality for activity items, which makes me think that this may be a conflict with another plugin.