In forum posts, usernames with characters that extend below the baseline ("g", "p", "q", "y", "_", etc.) are cut off at the bottom. Underscores in particular are rendered completely invisible because of this.
The property overflow: hidden; on the .forum-message .avatar-container .user-name selector is the cause of it, and moving it to the parent .forum-message .avatar-container would fix it. You might also want to give .forum-message .avatar-container .user-name the property white-space: nowrap; to prevent it from wrapping anything to a new line, assuming that's intended.
I'm suprised nobody has reported this yet.